Embryological, paleontological and comparative anatomical evidence of evolution. Comparative anatomy Comparative anatomy and paleontology
Rudiments- organs that were well developed in ancient evolutionary ancestors, and now they are underdeveloped, but have not completely disappeared yet, because evolution is very slow. For example, a whale has pelvic bones. In humans:
- body hair,
- third eyelid
- coccyx,
- muscle that moves the pinna,
- appendix and cecum,
- wisdom teeth.
Atavisms- organs that should be in a rudimentary state, but due to developmental disorders have reached a large size. A person has a hairy face, a soft tail, the ability to move the auricle, and multiple nipples. Differences between atavisms and rudiments: atavisms are deformities, and everyone has rudiments.
Homologous organs- externally different, because they are adapted to different conditions, but have a similar internal structure, since they arose from the same original organ in the process divergence. (Divergence is the process of divergence of characteristics.) Example: bat wings, human hand, whale flipper.
Similar bodies- externally similar, because they are adapted to the same conditions, but have a different structure, because they arose from different organs in the process convergence. Example: the eye of a person and an octopus, the wing of a butterfly and a bird.
Convergence is the process of convergence of characteristics in organisms exposed to the same conditions. Examples:
- aquatic animals of different classes (sharks, ichthyosaurs, dolphins) have a similar body shape;
- Fast running vertebrates have few fingers (horse, ostrich).
1. Establish a correspondence between an example of an evolutionary process and the ways in which it is achieved: 1) convergence, 2) divergence. Write numbers 1 and 2 in the correct order.
A) the forelimbs of a cat and the upper limbs of a chimpanzee
B) a bird's wing and a seal's flippers
B) an octopus tentacle and a human hand
D) penguin wing and shark fins
D) different types of mouthparts in insects
E) butterfly wing and bat wing
Answer
2. Establish a correspondence between the example and the process of macroevolution that it illustrates: 1) divergence, 2) convergence. Write numbers 1 and 2 in the order corresponding to the letters.
A) the presence of wings in birds and butterflies
B) coat color in gray and black rats
B) gill breathing in fish and crayfish
D) different shapes of beaks in great and tufted tits
D) the presence of burrowing limbs in moles and mole crickets
E) streamlined body shape in fish and dolphins
Answer
3. Establish a correspondence between animal organs and the evolutionary processes as a result of which these organs were formed: 1) divergence, 2) convergence. Write numbers 1 and 2 in the order corresponding to the letters.
A) limbs of a bee and a grasshopper
B) dolphin flippers and penguin wings
B) bird and butterfly wings
D) the forelimbs of a mole and a mole cricket insect
D) limbs of a hare and cat
E) the eyes of a squid and a dog
Answer
4. Establish a correspondence between animal organs and the evolutionary processes as a result of which these organs were formed: 1) convergence, 2) divergence. Write numbers 1 and 2 in the order corresponding to the letters.
A) limbs of a mole and a hare
B) butterfly and bird wings
B) eagle and penguin wings
D) human nails and tiger claws
D) gills of crab and fish
Answer
Choose one, the most correct option. The development of a small number of digits in the limbs of the horse and ostrich is an example
1) convergence
2) morphophysiological progress
3) geographical isolation
4) environmental insulation
Answer
Choose one, the most correct option. An example of a vestigial organ in humans is
1) cecum
2) multi-nipple
3) gill slits in the embryo
4) scalp
Answer
Choose three correct answers out of six and write down the numbers under which they are indicated. Rudiments include
1) human ear muscles
2) belt of the hind limbs of the whale
3) underdeveloped hair on the human body
4) gills in embryos of terrestrial vertebrates
5) multiple nipples in humans
6) elongated fangs in predators
Answer
Choose one, the most correct option. As a result of what evolutionary process aquatic animals of different classes (sharks, ichthyosaurs, dolphins) acquired a similar body shape
1) divergence
2) convergence
3) aromorphosis
4) degeneration
Answer
Choose one, the most correct option. Which pair of aquatic vertebrates supports the possibility of evolution based on convergent similarities?
1) blue whale and sperm whale
2) blue shark and bottlenose dolphin
3) fur seal and sea lion
4) European sturgeon and beluga
Answer
Choose one, the most correct option. The development of limbs of different structures in mammals belonging to different orders is an example
1) aromorphosis
2) idioadaptations
3) regeneration
4) convergence
Answer
Look at a picture of wings on different animals and determine: (A) what evolutionists call these organs, (B) what group of evolutionary evidence these organs belong to, and (C) what mechanism of evolution resulted in their formation.
1) homologous
2) embryological
3) convergence
4) divergence
5) comparative anatomical
6) similar
7) driving
8) paleontological
Answer
Establish a correspondence between examples of objects and methods of studying evolution in which these examples are used: 1) paleontological, 2) comparative anatomical. Write numbers 1 and 2 in the correct order.
A) cactus spines and barberry spines
B) remains of beast-toothed lizards
B) phylogenetic series of the horse
D) multiple nipples in humans
D) human appendix
Answer
Choose one, the most correct option. What sign in a person is considered an atavism?
1) grasping reflex
2) the presence of an appendix in the intestine
3) abundant hair
4) six-fingered limb
Answer
1. Establish a correspondence between the example and the type of organs: 1) Homologous organs 2) Similar organs. Write numbers 1 and 2 in the correct order.
A) Forearm of a frog and a chicken
B) Mouse legs and bat wings
B) Wings of a sparrow and wings of a locust
D) Whale fins and crayfish fins
D) Burrowing limbs of moles and mole crickets
E) Human hair and dog fur
Answer
2. Establish a correspondence between the forms of adaptation of organisms to their environment and the organs that they have formed: 1) homologous, 2) similar. Write numbers 1 and 2 in the order corresponding to the letters.
A) streamlined shape of the head of a shark and dolphin
B) owl wing and bat wing
C) a horse’s limb and a mole’s limb
D) human eye and octopus eye
D) carp fins and fur seal flippers
Answer
Establish a correspondence between the characteristics of organs and comparative anatomical evidence of evolution: 1) homologous organs, 2) similar organs. Write numbers 1 and 2 in the order corresponding to the letters.
A) lack of genetic relatedness
B) performing various functions
B) a single plan for the structure of five-fingered limbs
D) development from identical embryonic rudiments
D) formation under similar conditions
Answer
1. Establish a correspondence between the example and the sign: 1) rudiment, 2) atavism. Write numbers 1 and 2 in the order corresponding to the letters.
A) wisdom teeth
B) multi-nipple
B) muscles that move the auricle
D) tail
D) highly developed fangs
Answer
2. Establish a correspondence between the evolutionary characteristics of humans and their examples: 1) rudiment, 2) atavism. Write numbers 1 and 2 in the order corresponding to the letters.
A) muscles of the auricle
B) caudal vertebrae
B) facial hair
D) outer tail
D) vermiform appendix of the cecum
Answer
3. Establish a correspondence between the structural features of the human body and comparative anatomical evidence of its evolution: 1) atavisms, 2) rudiments. Write numbers 1 and 2 in the order corresponding to the letters.
A) folds of the nictitating membrane
B) accessory pairs of mammary glands
B) continuous hair on the body
D) underdeveloped ear muscles
D) appendix
E) caudal appendage
Answer
4. Establish a correspondence between the structures of the human body and evidence of evolution: 1) rudiment, 2 atavism. Write numbers 1 and 2 in the order corresponding to the letters.
A) ear muscles
B) appendix
B) coccygeal vertebrae
D) thick hair all over the body
D) multiple nipples
E) the rest of the third century
Answer
Consider the drawing depicting the inhabitants of the waters of different classes of vertebrates and determine (A) what type of evolutionary process the picture illustrates, (B) under what conditions this process occurs and (C) what results it leads to. For each lettered cell, select the appropriate term from the list provided. Write down the selected numbers in the order corresponding to the letters.
1) homologous organs
2) convergence
3) occurs in related groups of organisms that live and develop in heterogeneous environmental conditions
4) vestigial organs
5) occurs in the same conditions of existence of animals belonging to different systematic groups, which acquire similar structural features
6) similar bodies
7) divergence
Answer
Choose two correct answers out of five and write down the numbers under which they are indicated. The terms of evolutionary teaching include
1) divergence
2) monitoring
3) natural selection
4) plasmid
5) panspermia
Answer
Read the text. Select three sentences that indicate comparative anatomical methods for studying evolution. Write down the numbers under which they are indicated in the table. (1) Similar organs indicate the similarity of adaptations to the same environmental conditions in different organisms that arise during evolution. (2) Examples of homologous organs are the forelimbs of a whale, mole, and horse. (3) Rudiments are laid down during embryogenesis, but do not fully develop. (4) Embryos of different vertebrates within a phylum have a similar structure. (5) Currently, phylogenetic series have been compiled for elephants and rhinoceroses.
Answer
© D.V. Pozdnyakov, 2009-2019
Back in the first half of the 19th century. a number of data were obtained indicating the unity of the entire organic world. These include the discovery of the cellular structure of plants, animals and humans. The outstanding French zoologist J. Cuvier established uniform structural plans in each type of animal.
Comparative anatomical evidence of evolution
All vertebrates have bilateral symmetry, a body cavity, a spine, a skull, and two pairs of limbs. The heart of all vertebrates is located on the ventral side, and the nervous system is on the dorsal side, it consists of the brain and spinal cord. The unity of the building plan in each type indicates the unity of its origin.
Bilateral symmetry - the left half of the body is a reflection of the right
Homologous organs
After the publication of Darwin's works, comparative anatomy received an impetus for development and, in turn, made a significant contribution to the development of Darwinism.
Establishing the homology of organs played an important role. Homologous organs can perform different functions and, therefore, differ somewhat in structure, but are built according to the same plan and develop from the same embryonic rudiments.
These are the forelimbs of all vertebrates: the leg of a rabbit, the wing of a bat, the flipper of a seal, the hand of a person. The skeleton of each of these organs has a shoulder, a forearm consisting of two bones, a carpal bone, a metacarpus and a phalanges of the fingers. The same applies to the hind limbs. It was found that the mammary glands are homologous to sweat glands, the jaws of crustaceans are their limbs, the hair of mammals is homologous to the feathers of birds and the scales of reptiles, the teeth of mammals are homologous to the scales of sharks, parts of a flower (pistil, stamens, petals) are similar to leaves, etc.
Unlike homologous similar bodies may be similar in structure, since they perform homogeneous functions, but do not have a common structural plan of common origin. Examples of these include insect wings, bird wings, crustacean gills, and fish gills. In plants, cactus spines (modified leaves) and rose thorns (outgrowths of the skin) are similar. They do not play a role in establishing related relationships between organisms.
Atavisms and rudiments
To prove evolution matters atavistic organs, which were inherent in distant ancestors and are not normally found in modern organisms. Naturally, such features indicate phylogenetic relationship. Examples of atavism are the appearance of lateral toes in a horse, striping in domestic pigs; cervical fistula (formation homologous to gill slits in lower chordates), caudal appendage, profuse hairiness of the entire body in humans.
Vestigial These are organs that have lost their function but remain in adult animals. They usually remain in their infancy. The remains of the pelvic bones are vestigial in the legless yellow-bellied lizard and in cetaceans. They serve as evidence of the origin of these animals from ancestors who had developed limbs. In humans, the vestigial organs are:
- The coccyx is the remnant of the caudal vertebrae;
- rudimentary ear muscles indicating that human ancestors had a movable auricle.
On the rhizomes of fern, wheatgrass, and lily of the valley, you can find scales - rudiments of leaves.
Comparative anatomical studies of modern progressive and primitive forms make it possible to detect transitional forms. The marine animal Balanogloss combines the characteristics of animals such as echinoderms and chordates. The lancelet has a number of characteristics that bring it closer on the one hand to echinoderms and hemichordates (balanoglossus), and on the other hand to vertebrates, with which it belongs to the same type of chordates.
Among modern mammals, there are monotremes (which have a cloaca and lay eggs during reproduction, like reptiles), marsupials and placentals. Comparison of them indicates that mammals are related to reptiles and that the evolution of mammals went from animals that lay eggs, to viviparous forms with a still underdeveloped placenta, and, finally, to animals that give birth to well-formed young.
Embryological evidence for evolution
Even before the publication of Darwin’s main work, Academician of the Russian Academy of Sciences K.M. Baer established that the embryos of various animals are more similar to each other than the adult forms. Darwin saw this pattern as important evidence of evolution. He believed that in embryonic development the characteristics of ancestors should be repeated.
In the post-Darwinian period, the connection between ontogenesis and phylogeny was confirmed by numerous studies. Russian scientists A.O. Kovalevsky and I.I. Mechnikov established that in all multicellular organisms (invertebrates, starting with worms and vertebrates), three germ layers are formed, from which all organs are subsequently formed. This confirms the unity of origin of the entire animal world.
A comparison of the development of embryos of all classes of vertebrates shows their great similarity in the early stages of development, it concerns both external and internal structure (notochord, organs of the circulatory and excretory systems). As development progresses, the similarity decreases, and signs of a class, then order, genus and species begin to emerge. This confirms the relationship of all chordates.
Based on embryological studies carried out on objects from various types of animals, F. Muller and E. Haeckel (independently of each other) formed the biogenetic law.
The condensed formulation of the biogenetic law reads: ontogeny is a brief repetition of phylogeny.
Further embryological studies showed that the biogenetic law is valid only in general terms. In fact, there is not a single stage of development in which the embryo completely repeats the structure of any of its ancestors. The embryo of a bird or mammal never completely replicates the structure of a fish, but at a certain stage of development it develops gill slits and gill arteries. In ontogenesis, the structure of the embryos, rather than the adult forms of the ancestors, is repeated. In mammalian embryos, it is not the gill apparatus of adult fish that is formed, but only the anlage of the gill apparatus of fish embryos.
It has been established that in embryonic development not only organs associated with the repetition of characteristics are formed, but also temporary organs that ensure the existence of embryos in the conditions in which they undergo development.
Academician A.N. Severtsov clarified and supplemented the provisions of the biogenetic law. He proved that in the process of ontogenesis there is a loss of certain stages of historical development, a repetition of the embryonic stages of the ancestors, and not adult forms, and the occurrence of changes and mutations that the ancestors did not have. New hereditary characteristics that change the structure of the adult organism and the direction of evolution appear at different periods of embryonic development. The later in the process of embryonic development new characteristics arose, the more fully the biogenetic law manifests itself.
Paleontological evidence of evolution
Darwin believed that it was paleontology, the study of the fossil remains of the Earth's former inhabitants, that should provide the most compelling evidence in favor of evolution. Darwin was acutely aware of the lack of information about transitional forms, fossil organisms that combine the characteristics of ancient and younger groups belonging to different classes and types.
Evidence of evolution using the horse as an example
The first most compelling paleontological evidence of evolution was obtained by V.O. Kovalevsky (1842-1883). He managed to figure out the successive stages of the origin of equids, to which the horse belongs. The oldest ancestor of the horse, found in sediments of the Tertiary period, was about 30 cm high, had four toes on the front limbs and three on the hind limbs. He moved, relying on all the phalanges of his fingers, which was an adaptation to living in swampy areas. His food consisted of fruits and seeds.
Further, due to climate change, forests became less and less and at the next stage of evolution, the ancestors of the horse found themselves in open areas such as steppes. This led to the survival of those capable of fast running (to escape from predators), which was achieved by lengthening the limbs and reducing the support surface, i.e. reducing the number of fingers in contact with the soil.
At the same time, selection was aimed at adapting to feeding on steppe grasses. Folded teeth appeared with a large chewing surface necessary for grinding tough plant foods. Consistently, the middle finger became larger and larger, and the side fingers became smaller and smaller. As a result, the fossil horse, like the modern one, had only one toe on each leg, on the tip of which it rested. The height has increased to 150 cm. The entire body structure is well adapted for living in open steppe areas.
Other transitional forms
After research by V.O. Kovalevsky, it was possible to establish the phylogenetic series of many other animals: proboscis, carnivores, mollusks.
Currently, the geological history of the Earth has been studied in some detail. It is known that in the most ancient layers remains of various types of invertebrates are found, and only in later layers do remains of vertebrates appear. It has been established that the younger the layers, the closer the remains of plants and animals are to modern ones.
Transitional forms have also been discovered. An important find was Archeopteryx, a first bird that retains a number of reptile characteristics. Signs of a bird:
- general form;
- the presence of feathers;
- resemblance of the hind limbs to the tarsus.
Signs of reptiles:
- Presence of caudal vertebrae;
- teeth;
- abdominal ribs.
A transitional form between reptiles and mammals has been found - wild-toothed lizards (theriodonts), which are similar to mammals in the structure of the skull, spinal column, and limbs. If in reptiles all the teeth are of the same type, then in theriodonts there is a differentiation of the teeth into incisors, canines, and molars, which gave rise to calling these fossil lizards animal-toothed.
In the fossil state, seed ferns were found, combining some of the characteristics of ferns and some of gymnosperms. This serves as evidence of the origin of seed plants from pteridophytes.
COMPARATIVE ANATOMY
also called comparative morphology, is the study of the patterns of structure and development of organs by comparing different types of living beings. Data from comparative anatomy are the traditional basis of biological classification. Morphology refers to both the structure of organisms and the science of it. We are talking about external signs, but internal features are much more interesting and important. Internal structures are more numerous, and their functions and relationships are more significant and diverse. The word "anatomy" is of Greek origin: the prefix ana with the root tom means "cutting". Initially, this term was used only in relation to the human body, but now it is understood as a branch of morphology that deals with the study of any organisms at the level of organs and their systems. All organisms form natural groups with similar anatomical characteristics of the individuals within them. Large groups are successively divided into smaller ones, the representatives of which have an increasing number of common features. It has long been known that organisms of a similar anatomical structure are similar in their embryonic development. However, sometimes even significantly different species, such as turtles and birds, are almost indistinguishable in the early stages of individual development. Embryology and the anatomy of organisms are so closely correlated that taxonomists (specialists in the field of classification) use data from both of these sciences equally when developing schemes for the distribution of species into orders and families. This correlation is not surprising, since anatomical structure is the end result of embryonic development. Comparative anatomy and embryology also serve as the basis for the study of evolutionary lineages. Organisms that descended from a common ancestor are not only similar in embryonic development, but also successively go through stages in it that repeat - although not with absolute accuracy, but in general anatomical features - the development of this ancestor. As a result, comparative anatomy is critical to understanding evolution and embryology. Comparative physiology also has its roots in and is closely related to comparative anatomy. Physiology is the study of the functions of anatomical structures; the stronger their similarity, the closer they are in their physiology. Anatomy usually refers to the study of structures that are large enough to be visible to the naked eye. Microscopic anatomy is usually called histology - this is the study of tissues and their microstructures, in particular cells. Comparative anatomy requires dissection (dissection) of organisms and deals primarily with their macroscopic structure. Although it studies structures, it uses physiological data to understand the connections between them. Thus, in higher animals there are ten physiological systems, the activity of each of which depends on one or more organs. Below, these systems are considered sequentially for animals of different groups. First of all, external features are compared, namely the skin and its formations. The skin is a kind of “jack of all trades”, performing a wide variety of functions; in addition, it forms the outer surface of the body, therefore it is largely accessible to observation without opening it. The next system is the skeleton. In mollusks, arthropods and some armored vertebrates it can be either external or internal. The third system is the musculature, which provides skeletal movement. The nervous system is placed in fourth place, since it is it that controls the functioning of the muscles. The next three systems are digestive, cardiovascular and respiratory. All of them are located in the body cavity and are so closely interconnected that some organs function simultaneously in two of them or even in all three. The excretory and reproductive systems of vertebrates also use some common structures; they are placed in 8th and 9th places. Finally, a comparative analysis of the endocrine glands that form the endocrine system is given. Comparisons of other glands, such as skin glands, are made as the organs in which they are located are considered.
PRINCIPLES OF COMPARATIVE ANATOMY
When comparing animal structures, it is useful to consider some general principles of anatomy. Among them, the following are considered especially important: symmetry, cephalization, segmentation, homology and analogy.
Symmetry refers to the features of the arrangement of body parts in relation to any point or axis. In biology, there are two main types of symmetry - radial and bilateral (bilateral). In radially symmetrical animals, such as coelenterates and echinoderms, similar body parts are arranged around a center, like spokes in a wheel. Such organisms are inactive or generally attached to the bottom, and feed on food objects suspended in the water. With bilateral symmetry, its plane runs along the body and divides it into mirror-like right and left parts. The dorsal (upper, or dorsal) and ventral (lower, or ventral) sides of a bilaterally symmetrical animal are always clearly distinguished (however, the same is true for forms with radial symmetry). Cephalization is the dominance of the head end of the body over the tail. The head end is usually thickened, located in front of a moving animal and often determines the direction of its movement. The latter is facilitated by the sensory organs almost always present on the head: eyes, tentacles, ears, etc. The brain, mouth opening, and often the animal’s means of attack and defense are also associated with it (bees are a well-known exception). In addition, it has been shown that physiological processes (metabolism) occur here more intensively than in other parts of the body. As a rule, the separation of the head is accompanied by the presence of a tail at the opposite end of the body. In vertebrates, the tail was originally a means of locomotion in water, but over the course of evolution it began to be used in other ways. Segmentation is characteristic of three types of animals: annelids, arthropods and chordates. In principle, the bodies of these bilaterally symmetrical animals consist of a number of similar parts - segments, or somites. However, although the individual rings of an earthworm are almost identical to each other, there are differences even between them. Segmentation can be not only external, but also internal. In this case, the organ systems within the body are divided into similar parts, arranged in rows in accordance with the externally noticeable boundaries between the somites. Segmentation of chordates appears to be genetically unrelated to that observed in worms and arthropods, but arose independently during evolution. Bilateral symmetry, cephalization and segmentation are characteristic of animals that move quickly in water, on land and in the air.
Homology and analogy. Homologous animal organs have the same evolutionary origin, regardless of the function performed in a given species. These are, for example, human hands and bird wings or the tails of fish and monkeys, which are the same in origin, but are used differently. Similar structures are similar in their functions, but have different evolutionary origins. These are, for example, the wings of insects and birds or the legs of spiders and horses.
The dermis is the thick and relatively soft inner tissue of the skin. It is formed from the middle germ layer, mesoderm, provides nutrition to the epidermis, contains nerve endings, blood vessels, and is often rich in fatty deposits. The bases of hair and feathers are also located here, as well as glands, which are invaginations of the epidermis. Typically, the skin fits more or less loosely around the body and is separated from the underlying structures by a layer of loose connective tissue - subcutaneous tissue, containing many intercellular spaces. Arthropods have an external skeleton formed by ectoderm cells. Its outer layer is periodically shed due to body growth. In mollusks, the soft and often ciliated ectoderm usually secretes a protective calcareous shell. The first animal in the evolutionary series with real skin is the lancelet. Its epidermis is formed by a single layer of densely packed cubic cells; however, the cells of the dermis degenerate and fuse, so that it appears structureless and the skin as a whole appears single-layered.
Fish. The skin of fish contains many mucous glands and is usually covered with numerous scales. Several types are known. The scales of sharks and similar forms develop like teeth and are called placoid. The scales of modern bony fishes are formed from the inner layer of skin and are ctenoid (toothed, comb-like) or cycloid (round).
The scale primordium is a calcareous deposit in the dermis layer. As it grows, its edge extends out through the epidermis, so that the scales overlap each other like tiles. In some fish, such as the American shell pike, the scales do not overlap each other, but cover the body like tiles. They are called ganoids and increase in size as the fish grows. On cycloid and ganoid scales, seasons of intense growth leave layers resembling tree rings.
Amphibians. The skin of these animals is an additional respiratory organ: it is soft, moist and equipped with a dense network of blood vessels. It contains a huge number of mucous and poisonous glands; characterized by local accumulations of pigment, creating a camouflage color. All amphibians shed their outer layer of skin in a single layer as they grow. At least in the very early stages of development of aquatic amphibian larvae, their ectoderm cells bear cilia that facilitate locomotion and respiration. Keratin is first deposited in the outermost layer of the skin, preventing moisture loss through evaporation. However, amphibians have not yet made significant progress in terms of protection from desiccation and inhabit more or less humid places. The skin of some ancient amphibians contained large bony plates.
Reptiles. The main property of their skin is its ability to resist drying out. It is entirely covered with scales, hard and dry, which is associated with adaptation to life on land, but it can also be elastic, for example in lizards and snakes. In addition, it may contain bony plates, forming an armored covering, as in turtles or on the back and head of crocodiles. Snakes and lizards shed the outer layer of skin in a single layer, while in turtles it comes off in separate flaps. Reptiles have few skin glands. Scent glands are located on the chin and along the edges of the shell in some turtles, on the back of the thighs and around the cloaca in alligators and crocodiles, and near the cloaca opening in a number of snakes. Claws on the fingers first appear in some amphibians, but in them they do not play a significant role. All reptiles with limbs, except sea turtles, have well-developed claws.
Birds. The skin of birds cannot be called strong or dense, but it is rich in fat. There are few skin glands, but there is almost always a large sebaceous (coccygeal) gland above the base of the tail. Earwax glands may be located near the external opening of the ear. The feet of birds are covered with the same scales as those of reptiles. Their claws are also similar in origin.
Beak. The horny covers of the jaws of turtles and birds are formed by a modified outer layer of the epidermis. A similar beak was also characteristic of some extinct dinosaurs from the class of reptiles. Among birds, toucans shed their superficial horny layers, like reptiles' skin when molting. The beaks of birds vary in shape and size, which is associated with adaptation to a certain method of feeding. The forelimbs of birds are adapted for flight, so tasks usually performed by the hands of other animals are transferred to the beak. In addition, animals with beaks lack teeth. It can be used as a weapon, for cleaning feathers, for climbing, courtship, nest building, etc. Feathers are a derivative of reptile scales and a characteristic feature of bird skin. Like scales, a feather begins its growth in the form of a connective tissue protrusion (papilla) of the corium. However, it does not flatten, but stretches into a cylinder, which, rising above the epidermis, splits along one side and unfolds, forming beards along the free edges. There are three main types of feathers: contour, down and filament. Contour feathers cover the entire body and reach their greatest size on the wings and tail. Downy feathers protect chicks, and in adult birds they form a heat-insulating layer under the contour ones. Powdered down, characteristic of herons and a number of other birds, is distinguished by fragile beards that crumble into powder used in cleaning the plumage. The filament feathers are located along with the down feathers under the contour feathers and can protrude to the surface near the corners of the mouth, forming sensitive hairs. For example, the fringed beard of a turkey is made up of thread-like feathers.
A typical contour feather includes 6 components: the quill, which is immersed in the skin and secures the feather in it; the rod, which is a continuation of the edge and the main axis of the feather; a flat fan of beards connected to each other; an additional feather extending near the junction of the rod and the rim; lower navel - hole at the base of the eye; the superior umbilicus is the second opening at the base of the accessory feather, allowing air to flow in and out of the hollow shaft.
Mammals. In mammals, the skin is usually quite loosely connected to the body by a thick and elastic layer of subcutaneous tissue. It contains numerous glands, such as milk, sebaceous, sweat and odorant. The glands of the last three categories can be very numerous. Mammary glands, characteristic of mammals, are large structures that serve to feed their young. They are usually located in two rows on the sides of the underside of the body, but can be grouped between the hind limbs, as in cows, horses and many other herbivorous animals, or placed in front, at chest level, as in elephants, monkeys and humans. Hair represents the second unique feature of mammalian skin. Hair is absent only in some of their aquatic forms, for example whales and sirens (the latter have developed facial setae). A number of animals, such as elephants and pangolins, have very sparse hair; Depending on the species, they vary in thickness - from the delicate fur of a beaver to the long quills of a porcupine. The hair serves for thermal insulation and protection from damage. In addition, hair may be specialized to perform specific functions; for example, on the muzzle of many animals there are tactile hairs (“whiskers”) called vibrissae.
Horns. In giraffes, deer and bovids, horns are bony outgrowths on the frontal bones of the skull, covered with skin or its derivatives. In giraffes they are constantly covered with skin, but in deer they branch out as they grow and eventually lose their skin. The horns of rhinoceroses and the scales of pangolins are formed by a mass of fused hairs. In bovids, such as cows and antelopes, as well as in the American pronghorn, the horns are covered with keratin (horny) sheaths, derivatives of the stratum corneum of the epidermis. Pronghorn has these sheaths, while deer have entire antlers shed each year and grow back.
Claws. In mammals, claws reach the pinnacle of their development and diversity. The nails of monkeys and humans and the hooves of large herbivorous animals are modified claws.
SKELETAL SYSTEM
The skeleton supports, protects and connects the animal's body parts. It comes in different types and is formed from different materials.
Invertebrates. Among the simplest, radiolarians have a complex, geometrically regular flint skeleton, and foraminifera are protected by calcareous shells of a peculiar shape. Sponge skeletons can be built from three different materials: lime, the horn-like protein spongin, and silica. Lime and spongin are sometimes combined, but glass sponges have a purely flint skeleton. In coelenterates, the skeleton is rare, except for corals, in which it is formed by both external and internal calcareous structures. Coral reef limestones are mainly deposits of the skeletons of dead corals. In all primitive groups, the skeleton plays a supporting and protective role, but is not used for locomotion. Flatworms and roundworms lack it. Some annelids live in calcareous tubes formed by their own secretions. Different types of worms have setae, which are considered skeletal structures. Calcareous shells of mollusks are mainly external formations; the exception is the inner shell of the cuttlefish. Slugs and octopuses lack skeletons. Arthropods are characterized by a composite skeleton that covers the outside of their entire body, including antennae (antennae) and legs. It consists of the carbohydrate chitin, and in crustaceans it can contain large amounts of calcium. The chitinous shell, which develops from the ectoderm during embryogenesis, is a dead formation and cannot grow, therefore, increasing in size, all arthropods periodically shed the outer layer of the skeleton (molt). Roundworms also repeatedly change their tough outer covering, called the cuticle, as they grow.
Vertebrates. The vertebrate skeleton is formed not only by bones: it includes cartilage and connective tissue, and sometimes it includes various skin formations. In vertebrates, it is customary to distinguish the axial skeleton (skull, notochord, spine, ribs) and the skeleton of the limbs, including their girdles (shoulder and pelvic) and free sections. Lancelets have a notochord, but no vertebrae or limbs. Snakes, legless lizards and caecilians lack the skeleton of limbs, although some species of the first two groups retain their rudiments. In eels, the pelvic fins corresponding to the hind limbs have disappeared. Whales and sirenians also have no external signs of hind legs.
Scull. Based on their origin, there are three categories of skull bones: replacement cartilage, integumentary (overhead, or skin) and visceral. Invertebrates lack a structure comparable to the skull of vertebrates. In hemichordates, tunicates and cephalochordates there are no signs of a skull. Cyclostomes have a cartilaginous skull. In sharks and their relatives, it may once have contained bones, but now its box is a single monolith of cartilage with no seams between the elements. Bony fish have more different types of bones in their skulls than any other class of vertebrates. In them, like all higher groups, the central bones of the head are embedded in cartilage and replace it, and therefore are homologous to the cartilaginous skull of sharks. Integumentary bones arise as calcareous deposits in the dermal layer of the skin. In some ancient fish, they were plates of shell that protected the brain, cranial nerves and sensory organs located on the head. In all higher forms, these plates migrated into the depths, were incorporated into the original cartilaginous skull and formed new bones, closely related to the replacement ones. Almost all of the outer bones of the skull come from the dermal layer of the skin. The visceral elements of the skull are derivatives of the cartilaginous gill arches that arose in the walls of the pharynx during the development of gills in vertebrates. In fish, the first two arches have changed and turned into the jaw and hyoid apparatus. In typical cases, they retain 5 more gill arches, but in some genera their number has decreased. The primitive modern sevengill shark (Heptanchus) has as many as seven gill arches behind the jaw and hyoid arches. In bony fishes, the jaw cartilages are lined with numerous integumentary bones; the latter also form gill covers that protect the delicate gill filaments. During the evolution of vertebrates, the original jaw cartilages were steadily reduced until they disappeared completely. If in crocodiles the remainder of the original cartilage in the lower jaw is lined with 5 paired integumentary bones, then in mammals only one of them remains - the tooth, which completely forms the skeleton of the lower jaw. The skull of ancient amphibians contained heavy integumentary plates and was similar in this respect to the typical skull of lobe-finned fish. In modern amphibians, both applique and replacement bones are greatly reduced. There are fewer of them in the skull of frogs and salamanders than in other vertebrates with a bony skeleton, and in the latter group many elements remain cartilaginous. In turtles and crocodiles, the skull bones are numerous and tightly fused to each other. In lizards and snakes they are relatively small, with the external elements separated by wide intervals, as in frogs or toads. In snakes, the right and left branches of the lower jaw are very loosely connected to each other and to the cranium by elastic ligaments, which allows these reptiles to swallow relatively large prey. In birds, the skull bones are thin but very hard; in adults they have fused so completely that several sutures have disappeared. The orbital sockets are very large; the roof of the relatively huge braincase is formed by thin integumentary bones; the light jaws are covered with horny sheaths. In mammals, the skull is heavy and includes powerful jaws with teeth. The remains of the cartilaginous jaws moved to the middle ear and formed its bones - the hammer and the incus.
In birds and reptiles, the skull is attached to the spine using one of its condyles (articular tubercle). In modern amphibians and all mammals, two condyles located on the sides of the spinal cord are used for this. The spine, or vertebral column, is present in all chordates, with the exception of the skullless and tunicates. In embryonic development, it is always preceded by a notochord, which is preserved for life in lancelets and cyclostomes. In fish, it is surrounded by vertebrae (in sharks and their closest relatives - cartilaginous) and looks clear-shaped. In mammals, only rudiments of the notochord are preserved in the intervertebral discs. The notochord is not transformed into vertebrae, but is replaced by them. They arise during embryonic development as curved plates that gradually surround the notochord in rings and, as they grow, almost completely displace it. A typical spine has 5 sections: cervical, thoracic (corresponding to the rib cage), lumbar, sacral and caudal. The number of cervical vertebrae varies greatly depending on the group of animals. Modern amphibians have only one such vertebra. Small birds can have as few as 5 vertebrae, while swans can have up to 25. The Mesozoic marine reptile plesiosaur had 72 cervical vertebrae. In mammals there are almost always 7 of them; the exception is sloths (from 6 to 9). In cetaceans and manatees, the cervical vertebrae are partially fused and shortened in accordance with the shortening of the neck (according to some experts, manatees have only 6 of them). The first cervical vertebra is called the atlas. In mammals and amphibians it has two articular surfaces, which include the occipital condyles. In mammals, the second cervical vertebra (epistropheus) forms the axis on which the atlas and skull rotate. Ribs are usually attached to the thoracic vertebrae. Birds have about five, mammals have 12 or 13; snakes have a lot. The bodies of these vertebrae are usually small, and the spinous processes of their upper arches are long and inclined backwards. There are usually from 5 to 8 lumbar vertebrae; in most reptiles and all birds and mammals they do not bear ribs. The spinous and transverse processes of the lumbar vertebrae are very powerful and, as a rule, directed forward. In snakes and many fish, the ribs are attached to all the trunk vertebrae, and it is difficult to draw the boundary between the thoracic and lumbar regions. In birds, the lumbar vertebrae are fused with the sacral vertebrae, forming a complex sacrum, which makes their back more rigid than that of other vertebrates, with the exception of turtles, in which the thoracic, lumbar and sacral regions are connected to the shell. The number of sacral vertebrae varies from one in amphibians to 13 in birds. The structure of the caudal region is also very diverse; in frogs, birds, apes and humans it contains only a few partially or completely fused vertebrae, and in some sharks it contains up to two hundred. Toward the end of the tail, the vertebrae lose their arches and are represented by only bodies.
Ribs first appear in sharks as small cartilaginous projections in the connective tissue between muscle segments. In bony fishes they are bony and homologous to the haemal arches located below on the caudal vertebrae. In four-legged animals, such fish-type ribs, called lower, are replaced by upper ones and are used for breathing. They are laid in the same connective tissue partitions between muscle blocks as in fish, but are located higher in the body wall.
Skeleton of limbs. The limbs of tetrapods developed from the paired fins of lobe-finned fish, the skeleton of which contained elements homologous to the bones of the shoulder and pelvic girdle, as well as the front and hind legs. Originally there were at least five separate ossifications in the shoulder girdle, but in modern animals there are usually only three: the scapula, the clavicle and the coracoid. In almost all mammals, the coracoid is reduced, attached to the scapula, or absent altogether. In some animals, the scapula remains the only functional element of the shoulder girdle. The pelvic girdle includes three bones: the ilium, the ischium and the pubis. In birds and mammals they completely merged with each other, in the latter case forming the so-called. innominate bone. In fish, snakes, whales and sirens, the pelvic girdle is not attached to the spine, which therefore lacks the typical sacral vertebrae. In some animals, both the shoulder and pelvic girdles include accessory bones. The bones of the front free limb in quadrupeds are basically the same as those in the hind limb, but they are called differently. In the forelimb, if you count from the body, first comes the humerus, followed by the radius and ulna, then the carpals, metacarpals and phalanges of the fingers. In the hind limb they correspond to the femur, then the tibia, tibia, tarsus, metatarsal bones and phalanges of the fingers. The initial number of fingers is 5 on each limb. Amphibians have only 4 toes on their front paws. In birds, the forelimbs are transformed into wings; the bones of the wrist, metacarpus and fingers are reduced in number and partially fused to each other, the fifth finger on the legs is lost. The horses only have their middle finger left. Cows and their closest relatives rest on the third and fourth toes, and the rest are lost or reduced. Ungulates walk on their toes and are called phalangeal walkers. Cats and many other animals, when walking, rely on the entire surface of their fingers and belong to the digitimate type. Bears and humans press their entire sole to the ground when moving and are called plantigrade walkers.
Exoskeleton. Vertebrates of all classes have an exoskeleton in one way or another. The head plates of scutes (extinct jawless animals), ancient fish and amphibians, as well as the scales, feathers and hair of higher tetrapods, are skin formations. The shell of turtles is of the same origin - a highly specialized skeletal formation. Their skin bone plates (osteoderms) moved closer to the vertebrae and ribs and merged with them. It is noteworthy that the shoulder and pelvic girdles parallel to this have shifted inside the chest. In the crest on the back of crocodiles and the shell of armadillos there are bone plates of the same origin as the shell of turtles.
MUSCULAR SYSTEM
The main function of the muscular system is to move parts of the skeleton; the corresponding muscles are called skeletal. However, there are other types and functions. By contracting, the muscles create a pulling force; they cannot push. At the same time, they become thicker and shorter, but their volume does not change noticeably. Muscle activity is controlled by the nervous system and can be voluntary or involuntary. Skeletal muscles are of the voluntary type.
Types of muscles. In vertebrates, there are three categories of muscle tissue: striated, cardiac and smooth. The striated muscles, which form the bulk of the body's tissue, act voluntarily. They are connected to the skeleton, contract with great speed and force, but with prolonged work they always get tired and require rest. By their nature they are segmental, and in color they can be red, like beef, or light (“white”), like in fish and in the “breast” of chickens. Their fibers are multinucleated and collected in bundles, surrounded by a connective tissue film called perimysium. Smooth muscles are not attached to the skeleton; they are located in the walls of blood vessels, the digestive tract and in the dermal layer of the skin. These muscles are devoid of transverse stripes, contract involuntarily, slowly and weakly, but do not know fatigue. Their cells are mononuclear and are not grouped into bundles surrounded by perimysium. In this respect they resemble the muscle cells of lower invertebrates. The heart muscle (myocardium) is formed by cells that develop from the same embryonic tissue as the smooth muscle cells of the blood vessels, but here they are multinucleated, red in color and capable of rapid and strong contraction. In lower vertebrates they are somewhat elongated, while in higher ones they are wide and connected by jumpers into a narrow-loop network.
Invertebrates. It is difficult to say when muscles arose during the evolution of the animal kingdom. Contractile fibers are found in the cells of protozoa, sponges, and coelenterates, but specialized muscle cells appear only in flatworms and roundworms. In all invertebrates up to the level of mollusks, they lack cross-striations and resemble the smooth muscle cells of vertebrates. They do not contract very strongly and always relatively slowly. The exception here is mollusks: the closing muscles in bivalves can be considered skeletal. Developed muscles are characteristic of annelids, especially earthworms. In the wall of their body there are circular muscles, which reduce its diameter, and longitudinal muscles, which shorten it. There are also microscopic muscles (there are 4 pairs of them in each body segment) that move the bristles and are capable of sticking them into the soil. The earthworm crawls in its characteristic way due to contractions of all three categories of muscles - circular, longitudinal and microscopic. Excellent striated muscles, capable of rapid and powerful contraction, are characteristic of arthropods. The flight muscles of some insects are the fastest-acting of all known: in this sense they surpass even the similar muscles of hummingbirds. It is interesting to note that the skeletal muscles of arthropods control the movements of the exoskeleton, being inside it, under its protection.
Vertebrates. Vertebrate muscles can be divided into five groups depending on their embryonic origin: segmental (skeletal), visceral, ocular, cutaneous, and branchiomeric muscles. The segmental muscles never cross the midline of the abdomen; they are located in overlapping layers on the sides of the body in accordance with the original segments, or somites, of the embryo. The muscles of the limbs also develop from these axial blocks. In lancelets, cyclostomes and fish, the segmental muscles remain in their original and most elementary state. In fish fins they are simple and consist mainly of lifters and lowerers. In the limbs of tetrapods they are numerous and varied in function. Segmental muscles are attached to the bones of the skeleton either directly or with the help of tendons (strands of connective tissue). Visceral muscles, acting involuntarily and devoid of transverse stripes, are located primarily in the walls of the digestive tube. They are responsible for the peristaltic movements that push food through the digestive tract. In the region of the pharynx in fish, their non-segmental blocks are attached to the gill arches and turn into the striated muscles of the branchiomeres. In higher vertebrates they extend onto the surface of the head, becoming voluntary facial and jaw structures. This is a remarkable example of the convergent transformation of involuntary smooth muscles into voluntary striated muscles in the process of their adaptation to the role of skeletal muscles.
Eye muscles. The mobility of the eyeballs is ensured by the fact that six thin muscles are attached to them. In all vertebrates they arise from three paired somites in the head of the embryo. By their origin, the ocular muscles are related to the segmental ones, but are usually considered separately because of their uniqueness. Their work is controlled by the third, fourth and sixth cranial nerves. The cutaneous muscles are very unique in origin. When segmental muscles arise from the middle germ layer, mesoderm, free cells are separated from its outer edge, losing their segmental distribution. They form a vaguely defined layer of tissue called the dermatome, which completely surrounds the developing body of the embryo, adjacent to the ectoderm from the inside. From it the corium is formed along with the muscles located in it. They should not be confused with those that cause, for example, the trembling of the skin on the shoulders of a horse that drives away flies: such skin movements are caused by voluntary muscles - derivatives of skeletal muscles, and the skin muscles themselves are involuntary. In birds, they are attached to the bases of feathers and, when contracted, raise them. Similar muscles make the hair on the body of animals stand up. So-called pimples “Goose bumps” in humans are also the result of contraction of involuntary skin muscles.
NERVOUS SYSTEM
To regulate and coordinate the activities of all parts of the body, evolutionarily advanced animals have a highly specialized nervous system. In low-organized forms it is arranged relatively simply.
Invertebrates. In sponges, sensory (“sensitive”) mechanisms are not localized in strictly defined cells of the body, i.e. They don't have a real nervous system. Specialized nerve cells (neurons) appear in coelenterates. In Hydra they form a homogeneous network serving all parts of the body. In sea stars, the mouth is surrounded by a nerve ring, from which nerve trunks of ectodermal origin extend into each of the five arms. In flatworms and annelids, the head contains a paired collection of nerve cells called a ganglion (nerve ganglion) and serves as a primitive brain. A paired nerve trunk also stretches from it along the lower side of the body. In the earthworm, its branches are united and form the abdominal nerve cord with the ganglia. In arthropods, the nervous system is basically the same, the brain is enlarged and divided into lobes, the ventral nerve trunk is shortened, and some of its ganglia are fused with each other.
Vertebrates differ from invertebrates in three important features of the central nervous system: it occupies a dorsal position, develops from the dorsal ectoderm of the embryo, and is represented by a tube. It is laid as a longitudinal groove along the midline of the back. Later, the edges of the groove rise, bend towards each other and connect into the neural tube. At the head end it swells and forms protrusions, which turn into various parts of the brain. The structural basis of the nervous system is the neuron. It consists of a compact cell body and sensory and motor processes extending from it. Sensory processes called dendrites are highly branched and conduct nerve impulses into the body of the neuron. Along motor fibers, axons, impulses travel from the neuron body to another cell.
The nervous system of vertebrates is usually divided into two parts - central and peripheral. The first consists of the brain and spinal cord; the second is from the cranial (cranial) nerves, spinal nerves and the autonomic nervous system.
Brain. In the lancelet, only the cavity at the anterior end of the neural tube is expanded, and there is no brain as such. In all vertebrates, it is divided into 5 sections: the terminal, intermediate, midbrain, hindbrain and medulla oblongata. The main components of the telencephalon are the olfactory lobes, responsible for the corresponding “feeling,” and the cerebral hemispheres, the main center of nervous coordination. The diencephalon connects the telencephalon to the midbrain. The parietal organ (parietal eye) and the pineal gland (epiphysis) extend from its dorsal surface, and the optic nerves form below it. The main parts of the midbrain are the paired optic lobes, especially important for lower vertebrates. The hindbrain forms the cerebellum, which lies on the dorsal side of the medulla oblongata, which is responsible for the coordination of movements. All cranial nerves after the fourth arise on the sides of the medulla oblongata in front of its transition to the spinal cord.
HUMAN BRAIN. Its 5 main sections are noted (in humans, the cerebral hemispheres overlap many other parts). The forebrain (end) brain is highlighted with sparse dots; it consists of the olfactory lobes and the cerebral hemispheres. The diencephalon is blackened; it connects the forebrain to the midbrain. The midbrain is highlighted with frequent dots; its main part consists of the paired optic lobes. The hindbrain consists of the cerebellum (shown as dark lines) and the pons. The medulla oblongata gradually passes into the spinal cord. The cerebral hemispheres - the center of conscious sensitivity, voluntary activity, memory and intelligence - as well as the cerebellum, which is responsible for the coordination of movements, increase in the course of evolution, and their structure becomes more complex. The large hemispheres of both horses and humans are covered with grooves and convolutions, significantly increasing the surface of their gray matter, or cortex - the “thinking organ” of the animal. All cranial nerves after the fourth arise from the medulla oblongata. During evolution, the relative size of the pituitary gland decreases. Fish and birds are characterized by powerful development of the cerebellum. The pineal gland, or pineal gland, is considered a rudiment of an additional organ of vision.
The brain of the Squalus shark is elongated in length, and its olfactory and visual lobes are noticeably prominent. The large hemispheres are small, which indicates low development of “intelligence”; The cerebellum, which is hollow inside, is relatively large. All actively swimming (pelagic) fish have large optic lobes and cerebellum, since these animals require good vision and fine coordination of movements. The same is true for birds. In amphibians, the cerebellum is very poorly developed. In salamanders, the optic lobes are almost invisible, but in frogs and toads they are large, and they see perfectly. The main feature of the brain of birds and mammals is the large and complex cerebral hemispheres. Mammals are also characterized by a large, massive cerebellum; its cavity, free in the lower forms of vertebrates, is here occupied by the branches of nerve fibers, forming a peculiar pattern in the section - the “tree of life”. The optic lobes are transformed into a pair of anterior tubercles called quadrigeminal and play a subordinate role in providing vision. Its main center moved in mammals to the occipital lobe of the cerebral hemispheres. In vertebrates, the spinal cord extends from the brain along the spinal canal formed by the upper (neural) arches of the vertebrae. A deep and narrow dorsal and shallower and wider abdominal slits run along its entire length. Paired spinal nerves extend from the lateral surfaces, also along its entire length. Each begins with two roots - dorsal and ventral, which then merge. The dorsal root carries a ganglion (nerve ganglion), while the ventral root does not have one. In lower vertebrates, both roots contain motor nerve fibers, and the dorsal one, in addition, contains sensory fibers. In mammals, the dorsal root is purely sensory, and the ventral root is motor.
The number of paired spinal nerves varies widely - from 10 in frogs to several hundred in snakes. In three places on each side of the body they are connected to each other into plexuses: cervical, brachial (at the level of the shoulder girdle) and sacral (in the pelvis). The interconnections of nerves within the plexuses are weak in fish, more developed in amphibians and reptiles, and extremely complex in mammals.
Cranial nerves. A typical cranial nerve originates from the brain and exits the skull through a small opening. It was traditionally believed that fish and amphibians have 10 pairs of such nerves, and reptiles, birds and mammals have 12. However, this generalization requires some corrections. In 1895, in front of the first, the terminal (terminal) nerve was discovered, which, as it turned out, is present in all vertebrates except birds. It was called zero to avoid confusion in the existing numbering system. The names and numbers of the cranial nerves are as follows: 0 - terminal, I - olfactory, II - visual, III - oculomotor, IV - trochlear, V - trigeminal, VI - abducens, VII - facial, VIII - auditory, IX - glossopharyngeal, X - vagus, XI - accessory, XII - sublingual. These nerves are serially homologous to the spinal nerve roots, but are more specialized. The thin terminal nerve is considered sensory. The olfactory sense determines sensitivity to odors (in proto-aquatic vertebrates, it reacts to odorous substances in the water and not in the air). The optic nerve is formed as an outgrowth of the brain and initially represents a branch of the neural tube. At its peripheral end is the retina of the eye, from which it transmits impulses to the brain. The third, fourth and sixth nerves are motor nerves that control the eye muscles. The trigeminal nerve, which combines sensory and motor functions, arises as two separate nerves that unite at the gasserian (lunar) ganglion. In fish it is divided into 4 main branches going to different parts of the head, and in reptiles, birds and mammals it is divided into three, which is why it is called trigeminal. The facial nerve, also mixed (motor and sensory), innervates the hyoid arch, jaws and lateral line organs on the surface of the head in fish. In its functions it is similar to the trigeminal, but is located more superficially. The sensory auditory nerve is connected to the inner ear. In higher terrestrial vertebrates, it is divided into two branches: the cochlear branch goes to the auditory receptors, and the vestibular branch goes to the vestibule and semicircular canals (vestibular apparatus), therefore it is also called the vestibular-cochlear branch. The nerve as a whole serves hearing and spatial orientation. The mixed glossopharyngeal nerve in fish innervates the region of the first gill slit. In higher vertebrates, its branches go to the tongue and pharynx. The large, also sensory-motor, vagus nerve, part of the parasympathetic nervous system, controls the branchial region behind the first slit and sends large branches to the internal organs, particularly the lungs and stomach. It arose as a result of the union of at least four spinal nerves, the roots of which moved forward - onto the medulla oblongata. During evolution, the motor accessory nerve separated from the vagus nerve, the branches of which go to the neck and shoulders. In snakes it degenerates. The hypoglossal nerve controls the muscles of the tongue. It has already been noted in sharks, but in other fish and amphibians the XI and XII nerves are unknown. The autonomic nervous system consists mainly of a paired chain of nerve ganglia that stretches along the dorsal side of the abdominal cavity. It is connected to the cranial nerves, to each spinal nerve near the junction of its roots, and to all internal organs. This involuntary (autonomous) system controls smooth muscles, cardiac muscle, iris and ciliary muscles of the eye, all glands, as well as cutaneous muscles associated with the roots of feathers and hair. It consists of two systems that are opposite in their action - parasympathetic and sympathetic. If any organ controlled by these nerves receives a stimulating signal from one of them, then the other inhibits its activity. This dual nervous control of the glands, blood vessels, heart, intestines and intrinsic muscles of the eye ensures the harmonious functioning of all organs of the body. The parasympathetic system is connected to three centers - in the midbrain and medulla oblongata and in the sacral region of the spinal cord, and the sympathetic system is connected to the spinal nerves along the entire spinal cord from the medulla oblongata to the sacral region. The autonomic nervous system of all vertebrates is structured similarly, but in higher forms it is more complex.
Sense organs. Everyone knows such sensory organs of different animals as antennas (antennae, ears), ears, nose and eyes. There are many others - bristles, statocysts, sensory bodies, chemoreceptor (taste) buds, etc. Vertebrates typically have five senses: vision, hearing, taste, smell and touch; however, they also have a sense of balance (body position in space) and a corresponding organ, represented by the three semicircular canals of the inner ear and extremely important, for example, for birds and fish. In pit snakes, there is a small depression in front of each eye where a thermoreceptor organ is located that senses heat from a distance. There are also so-called general (i.e. not associated with special organs) sensations: thirst, hunger, cold, pain, pressure, muscle and tendon feelings. In typical cases, sensory impulses reach the central nervous system either through the cranial nerves or through the dorsal roots of the spinal nerves, and from the internal organs through the fibers of the autonomic nervous system. The lateral line organs, represented by special canals in the skin on the head and body of fish, are clearly visible in the larvae of amphibians and their aquatic forms, but in all terrestrial vertebrates they have disappeared without a trace. The chemical sense organs - smell and taste - are not always easily distinguishable in aquatic vertebrates, but, as a rule, are located in the mouth and nasal cavity in terrestrial ones. In insects they are located in the antennae, and in some fish they are on the skin.
Eyes. In lower invertebrates these may be just slightly specialized pigment spots. Spiders usually have 8 simple eyes at the top of their heads. In millipedes, simple eyes form two clusters on the sides of the head. Crayfish, lobsters and crabs are characterized by two compound eyes, consisting of a large number of small “eyes”. Insects usually have three simple and two compound eyes, but many small forms lack simple eyes. In cephalopods and vertebrates, the eyes, despite their high specialization, are striking in their similarity. They arise from completely different embryonic rudiments, but in their final form they are almost identically structured, down to the level of the eyelids, pupils, irises, lenses, fluid media and retinas containing rods and cones; True, the optic nerves are no longer the same. This is a striking example of the convergence of similar structures.
Ears. Hearing organs appear in some insects in the form of eardrums on the body or legs and associated structures. The vertebrate ear is a dual sense organ - hearing and balance.
DIGESTIVE SYSTEM
The digestive system is the intestinal tube (digestive tract) with all its auxiliary parts. It is most developed in vertebrates, in which it consists of a mouth, followed by a pharynx, esophagus, stomach, intestines and anus or cloaca. In addition, their digestive system includes the salivary glands, liver and pancreas.
Invertebrates. In protozoa, so-called digestive vacuoles inside the cell. Ciliates have many of them, and they act like small stomachs. Sponges do not have formations comparable to a stomach or intestines. These animals feed on plankton, i.e. microscopic living creatures suspended in water, which are drawn into their body through numerous pores as a result of the beating of special flagella, the so-called. collar cells. In coelenterates, the body wall has only two layers - ectoderm and endoderm, and it can be compared to a two-layer sac. The inner layer, endoderm, lines the intestinal cavity in all animals more complex than sponges. Thus, coelenterates have a kind of stomach (or intestines), but the rest of the digestive organs are absent, except for the mouth corresponding to the blastopore. In the embryos of all animals, the blastopore is the primary opening leading to the digestive tract. In almost all invertebrates, with the exception of echinoderms and some small groups, it turns into a mouth opening. In echinoderms and chordates, the blastopore becomes the anus, and the oral opening breaks through the digestive system later. In echinoderms it occurs in the center of the body on its underside, and in chordates it occurs where the head develops. It seems that this change in mouth position indicates that the cephalic end of the body of invertebrates is homologous to the caudal end of chordates.
Fish. The digestive system of spiny sharks (Squalus) is a good illustration of a variant that is primitive for fish. The large mouth is located on the underside of the head. The teeth, which are modified placoid scales, form several successive rows. Their shape is adapted only for cutting prey, although the ability to grind food before swallowing is extremely advantageous. Many bony fish have long and pointed teeth, suitable only for catching and holding prey; Some species of this group are toothless, but there are also those armed with pressing-type teeth. It can hardly be said that sharks have a tongue, except for a rather loose fold of skin that covers the inside of the cartilaginous hyoid arch. In bony fishes, this arch can protrude from below into the oral cavity, but never forms a muscular structure. The shark's pharynx is an extended extension of the oral cavity. Its side walls are supported by five gill arches. All fish have 5 gill slits. Almost all sharks and their close relatives have a modified gill slit behind the eye, connected to the hyoid arch. This is the so-called spray: through it, water enters the pharynx, which then washes the gills, which is necessary if the mouth is busy with food. In all cartilaginous fish, not counting chimeras, each gill slits, including the squirter, opens on the lateral surface of the body behind the head. In chimeras and bony fishes, these openings are covered from the outside by an operculum. In almost all fish, the pharynx leads directly to the stomach, and it is difficult to talk about the presence of an esophagus here. Sharks have a J-shaped stomach and are relatively very large. Like many other fish, the inner surface of the wall of its cardial (head) section is lined with long multi-branched papillae. These glandular formations secrete powerful digestive juices necessary for animals that swallow their prey whole or in large pieces. When the stomach is free of contents, it collapses and the middle and lower zones of its internal surface form longitudinal folds. When the stomach stretches, they flatten. The shark's intestines are short, which is generally typical for carnivorous (meat-eating) animals, while in herbivorous forms it is long. In the short intestine, meat does not stay long, otherwise it would begin to rot. The pyloric valve (a slightly modified circular sphincter muscle) separates the stomach from the small intestine. Immediately behind it, the ducts of the gallbladder and pancreas flow into it. The short small intestine continues with a wide thick intestine, with a spiral fold inside, the so-called. spiral valve. This formation significantly increases the internal surface of the intestine and thereby the rate of absorption. The spiral valve is found in lampreys, sharks, lungfishes, ganoids and some primitive bony fishes. In the latter, the intestines are often elongated, highly convoluted and surrounded by layers of fat. In sharks, it ends in a large chamber, the cloaca, into which the ducts of the kidneys and reproductive organs open. The cloaca is characteristic of cartilaginous and lungfishes, amphibians, reptiles, birds, as well as primitive oviparous mammals. In typical bony fish and mammals, the intestinal and genitourinary tracts are separated from each other. Many bony fish have three such openings: for feces, urine and reproductive products. In all aspects of anatomy, amphibians occupy a transitional position between ancient pulmonary fish and reptiles. They are characterized by small, uniform teeth and a fleshy tongue. In frogs, toads and some tailed forms, it is sticky and is able to quickly be thrown out of the mouth to catch small insects. In tailless animals, it is attached to the anterior edge of the lower jaw and at rest lies in the mouth with its apex backwards. Such a tongue is thrown out passively - with a sharp opening of the mouth, and is retracted back due to the contraction of its muscles. In tailed amphibians, the tongue moves forward in a forward motion. The pharynx of amphibians is formed in the gill region, present in their aquatic larvae and adults of some aquatic species, but in terrestrial forms the gills disappear before reaching land. The stomach, like in fish, is almost not separated from the oropharyngeal cavity, and the esophagus is poorly defined. Salamanders have a long stomach that matches the shape of their body, and their intestines form loops and are slightly twisted into a spiral. In frogs and toads, the stomach is curved so that its posterior section is oriented approximately across the spine, as in many mammals, and the intestines are curled into a ball. Reptiles differ little from amphibians in their digestive system, except for the oral cavity. The large conical teeth of crocodiles are covered with a layer of enamel. In both crocodiles and lizards they are all the same in shape - such a system is called homodont (in mammals they are different and the dental apparatus is heterodont). The poisonous teeth of snakes are equipped with a longitudinal channel, or groove, and form something like an injection needle. Snakes and lizards are not capable of chewing. Crocodiles tear off pieces of prey, and turtles take bites. Some snakes have mouths so extensible (the jaws are connected by elastic ligaments) that they can swallow prey four times the diameter of their resting head. The snake's long and retractable forked tongue is very sensitive. It constantly thrusts out and retracts and vibrates in front of her nose when she is aroused. The chameleon has a long, sticky tongue that extends far out of its mouth to catch small prey. Turtles and crocodiles have short and fleshy tongues. All reptiles have a pronounced esophagus and stomach, followed by a long, coiled intestine. Birds have a specialized digestive system, partly due to the presence of a beak, which does not allow them to chew food: the jaws with teeth must be strong, and therefore heavy, which is incompatible with flight. The inner lining of the oral cavity is usually hard and dry, and there are few taste buds. The shape of the tongue varies greatly: it is often forked or serrated towards the rear end (this helps push food towards the esophagus). The pharynx is not clearly defined: this area is distinguished by a respiratory opening leading from it into the larynx. The esophagus is a long tube that almost always includes an extended area for storing food, the so-called. goiter. In geese, owls and some other birds, the entire posterior part of the esophagus is expanded, and we can say that either there is no goiter, or this entire expanded region corresponds to it. Pigeons are the only birds that can drink water with their heads lowered below the body thanks to the peristaltic movements of the esophagus, like in mammals. From the esophagus (crop), food enters the anterior section of the stomach, the glandular section that was previously mistakenly considered part of the esophagus. This is an extension of the digestive tube, in the thick walls of which there are glands that secrete gastric juice. This is followed by the gizzard (“belly button”), a unique anatomical formation. Its muscles are a derivative of the light, involuntary muscles of the intestinal wall, but due to their high activity they have become dark red and appear striated, although they retain their involuntary character. In granivorous birds, the muscular stomach is especially well developed and is lined on the inside with horn-like tissue that does not contain glands. In carnivores, its walls are weaker, and their lining is soft. It is believed that some dinosaurs also had a muscular stomach like a bird's. Birds of prey have a short intestine, while herbivores have a very long and convoluted intestine. Near its posterior end, a pair of hollow outgrowths extends, the so-called. caecum. In owls they are very extensive, in chickens they are represented by long tubes, and in pigeons they are rudimentary. Mammals are characterized by a diverse and highly efficient digestive system. First of all, their lips reached their highest development. They appear in amphibians, and, with the exception of turtles, birds and whales, steadily increase during the evolution of vertebrates, culminating in rodents in the form of their huge cheek pouches. Mammalian teeth can be almost identical and conical (like those of dolphins and other toothed whales), adapted only for grasping and holding prey, but, as a rule, they are heterogeneous and complex in structure. A typical animal tooth consists of a crown covered with a layer of enamel. Beneath it there is dentin, which continues into the root, which is surrounded by a layer of cement. In the center of the dentin there is a cavity containing the so-called. pulp - soft tissue with artery, vein and nerve. Typically, tooth growth stops after reaching a certain size, but the tusks of some animals, the incisors of rodents, and the molars of bulls and horses wear heavily at the apex of the crown and, in order to continue to function, grow continuously at the base, where dentin, cementum, and enamel are formed. The pulp cavity of the latter type of teeth is open (it is not closed in the root, which is actually absent). Such teeth are called hypsodont. Typically, mammals have two sets of teeth. The first, so-called milk ones fall out and are replaced by permanent ones. Sirens and toothed whales have only one set of teeth. Mammals are characterized by 4 types of teeth: incisors, canines, premolars (premolars) and molars (molars). The latter appear only once - in the second change of teeth. Canines are especially strongly developed in carnivores, absent in rodents, and small or absent in bovids, deer and horses. The molars and premolars of predatory animals have specialized cutting edges. In pigs and humans, the tops of these teeth are relatively flat and are used for crushing food. In bovids, elephants and horses, layers of enamel, dentin and cement form complex folds in the flat-topped grinding teeth. Here the outer layer of cementum not only surrounds the root but also extends to the apex of the crown. The tongue in mammals develops mainly from a tubercle at the bottom of the pharynx. It grows forward and combines with other tissues in the area to form a complex and multifunctional muscular structure. This is a good organ of touch and the main area where taste buds are located. Usually the tongue is flattened and moderately stretchable. In anteaters it is round in cross section and can extend far from the mouth, like in woodpeckers; in whales it is almost motionless; in cats it is covered with horny papillae for scraping meat from bones. The esophagus stretches from the pharynx to the stomach in the form of a soft tube, varying slightly within the class. Food and liquids can be pushed through it by peristaltic muscle contractions. The relatively large stomach of mammals is usually located transversely in the anterior part of the abdominal cavity. Its anterior, cardiac end is wider than the posterior, pyloric end. The rest of the inner surface of the stomach wall, when unstretched, is folded, like in sharks and reptiles. In ruminants (cows, sheep, etc.) the stomach consists of four sections. The first three - scar, mesh and book - are derivatives of the esophagus, and the last - abomasum - corresponds to the stomach of most groups (according to some authors, the esophagus gave rise only to the scar and mesh). Ruminants eat quickly, filling a huge rumen with food, from which individual portions of cud are then formed in the mesh. Each of them is regurgitated, thoroughly chewed again and swallowed again, this time ending up in a book, from where it is sent to the abomasum and further to the intestine.
In mammals, the small and large intestines are clearly distinguishable. In typical cases, the first consists of three parts: duodenum, jejunum and ileum. The duodenum is so named because its length in humans approximately corresponds to the total width of 12 fingers (20-30 cm). The human jejunum is approximately 2.4 m long, and the ileum is approx. 3.4 m. There are no clear boundaries between these departments. In the jejunum, food is mainly digested, and in the ileum, absorption occurs. The large intestine consists of the cecum, colon and rectum; the latter ends with the anus. The cecum is a hollow outgrowth at the beginning of the large intestine. This variable formation, characteristic of mammals, was not inherited by them from reptilian ancestors, but developed during the evolution of the class as a place of accumulation of food that requires particularly long digestion. The cecum reaches its largest size in primitive herbivorous forms, which are characterized by its large hollow protrusion - a vermiform appendix (appendix). For a rabbit, this is a sac 36 cm long; in a pig, the blind tube is 90 cm long; in humans the appendix is vestigial; the cat does not have it. The ileum is located at a right angle to the cecum. The main function of the colon is to retain the remains of digested food and remove as much water as possible from them. The rectum is always represented by a short straight tube that ends in the anus, surrounded by two rings of sphincter muscles. The first works involuntarily, the second - voluntarily.
VASCULAR SYSTEM
The typical vascular system in higher groups of animals consists of two parts - circulatory and lymphatic. In the first of them, blood pumped by the heart circulates through a closed network of tubes (blood vessels - arteries, capillaries and veins): arteries carry blood from it, veins - to it. The lymphatic system includes lymph vessels, sacs, and glands (nodes). Lymph is a colorless liquid, similar in composition to blood plasma. Its source is liquid filtered through the walls of blood capillaries. It circulates in the intercellular spaces, enters the lymphatic vessels, and through them into the general bloodstream. The vascular system supplies all organs with nutrition and oxygen, while simultaneously removing waste products from them. The walls of lymphatic capillaries are more permeable than those of blood capillaries, so some substances, such as proteins, enter the lymph and are transported by it, and not by blood.
Invertebrates. Circulation in one form or another is characteristic of all animals. In ciliates (protozoa), digestive vacuoles move in the cytoplasm in approximately circles (so-called cyclosis). Flagellar collar cells push water through the sponge's body, allowing for respiration and filtering out food particles. Coelenterates do not have a special circulation system, but their digestive cavities diverge through channels to all parts of the body. In Hydra and many other cnidarians, they even extend into the tentacles. Thus, the body cavity plays a dual role here - digestive and circulatory.
Nemerteans are the most primitive modern animals with a true vascular system. It consists of three blood vessels that stretch along the entire body. In echinoderms, blood simply washes the vast body cavities. Annelids are characterized by red blood and the organs that pump it (the heart). Invertebrates have red blood: a red respiratory pigment, hemoglobin, is dissolved in its plasma. Squids, octopuses and some other mollusks and crustaceans have a different respiratory pigment - hemocyanin (gives the blood a blue color). An excellent vascular system with a complex network of arteries and veins and a well-developed heart is characteristic of mollusks. Arthropods also have a blood-pumping organ, which can be called a heart, but their circulatory system is not closed: the blood freely washes the spaces, or sinuses, inside the body, and the vessels are poorly developed, especially in insects. In the latter, the tracheal network frees the blood from the function of gas exchange.
Vertebrates. Lancelets are the only representatives of chordates that lack a heart, but the general layout of their primitive circulatory system is typical of higher groups. In all vertebrates, the heart is located closer to the ventral side of the body. Blood is colored red by hemoglobin, which is contained in special cells (erythrocytes); plasma is colorless. Fish, with the exception of lungfish, are characterized by a two-chambered heart, consisting of an atrium and a ventricle. The ventricle pumps blood to the gills, where it becomes oxygenated and turns bright red (arterial). From there it flows to the head through the carotid arteries, and to the remaining parts through the dorsal aorta, which continues in the tail in the form of the caudal artery. Two pairs of large branches are separated from the aorta - the subclavian and iliac arteries. The former go to the pectoral fins and the body walls adjacent to them, the latter to the pelvic region and ventral fins. Other paired arteries supply blood to the back muscles, kidneys and reproductive organs. Unpaired arteries branching from the aorta go to the internal organs in the body cavity. The largest of them - the celiac - sends its branches to the swim bladder, liver, spleen, pancreas, stomach and intestines. The fact that the swim bladder in fish is supplied with blood differently from the lungs serves as an additional argument against recognizing these organs as homologous. Having passed through the capillaries of all organs of the body, except the gills and lungs, the blood, losing oxygen, becomes dark (venous). From the head it enters the atrium through two large anterior cardinal veins. In sharks, it first fills the large venous sinus located immediately in front of the atrium. Venous blood flowing from the body and fins enters it through four pairs of large veins: subclavian (from the shoulder girdle and pectoral fins), lateral abdominal (from the side walls of the body and abdominal fins), hepatic (from the liver) and posterior cardinal (from the back and kidney). In the abdominal cavity, the portal vein carries venous blood to the liver from the stomach, intestines and spleen. In fish, most of the blood from the tail vein passes through the kidneys on its way to the heart. As vertebrates evolve, less and less venous blood is sent to them. In amphibians it goes mainly to the liver. In mammals, venous blood from all parts of the body behind the shoulder girdle does not enter the kidneys, but moves directly to the heart through the posterior vena cava.
This is a large azygos vein that runs in the upper part of the abdominal cavity. It is absent in fish, with the exception of lungfish. In amphibians it is already well expressed and in the American proteus (Necturus) it functions along with the posterior cardinal veins. In tailless amphibians, reptiles, birds and mammals, the latter are reduced.
Heart. In typical fish, all the blood from their two-chambered heart is directed to the body through the gills. In lungfish and amphibians, after the appearance of the lungs, only part of the blood flows from the heart to the gills. In its upper left part a second atrium appears, receiving arterial (oxygen-rich) blood from the lungs; the heart becomes three-chambered. Its same structure is preserved in typical reptiles. However, in crocodiles a septum appears in the ventricle, dividing it into two parts, i.e. the heart turns into a 4-chambered one. It is the same in birds and mammals. In animals with a 4-chambered heart, blood, making a full circle around the body, passes through the heart twice. From the head and the region of the shoulder girdle, it enters the right atrium through one or two anterior vena cava, and from other organs through the posterior vena cava. From the right atrium, blood enters the right ventricle and travels through the pulmonary arteries to the lungs. It returns from them through the pulmonary veins to the left atrium, from there it is pushed into the left ventricle, and from there it is distributed throughout the body along the aorta and its branches.
Aortic arches. If we count the squirter as the first gill slits, then modern sharks have six of them. In a typical embryo of any vertebrate, six arterial arches appear from the aorta; thus, this number can be considered as the initial number for the entire group, although the lancelet larva has 19, and some sharks have more than six. Modern sharks as adults have 5 pairs of gill arteries, which branch from the abdominal aorta and go to the gills, carrying blood to them from the heart. However, from the gills to the dorsal aorta, blood flows only through 4 pairs of gill arteries (the anterior one directs it to the head). In its middle part, each arterial arch breaks up into gill capillaries, dividing it into the afferent and efferent gill arteries. In typical bony fishes, only 4 pairs of aortic arches lead to the gills; there are the same number of efferent branchial arteries flowing into the dorsal aorta. In amphibians that retain gills, the first 3 of 6 arches are involved in the development of the internal and external carotid arteries. The same is observed in all higher animals, although in a greatly modified form. The fourth arches are large vessels that are the same on both sides of the body in amphibians, but different in reptiles. Birds do not develop a left aortic arch, while mammals do not develop a right one. The fifth arch disappeared along with the gill in adult frogs and toads. It is also absent in adult reptiles, birds and mammals. The outer end of the sixth arch also disappeared in almost all tetrapods, and its inner (closest to the heart) section turned into the pulmonary artery. In snakes, the left pulmonary artery is small or absent. In lungfishes and amphibians with gills, the pulmonary artery branches off from the preserved sixth arch.
RESPIRATORY SYSTEM
The main function of the respiratory system is to provide the body with oxygen and remove one of the oxidation products from it - carbon dioxide (carbon dioxide).
Invertebrates. Protozoa breathe across the entire surface of the cell. Coelenterates and sponges also lack a specialized respiratory system. Some annelids use gills, but generally they do not have respiratory structures. The body of some echinoderms is covered with numerous small dermal gills. Mollusks breathe either through gills or pulmonary sacs. Insects are characterized by tracheal tubes that penetrate their entire body. Crustaceans breathe through gills. Spiders use the so-called to breathe. pulmonary books with leaf-like gas exchange structures.
Vertebrates can breathe through gills, lungs, and through the surface of the skin.
Their gills are soft, thread-like outgrowths, abundantly washed with blood, in the wall of the gill slits leading from the pharynx to the sides of the body. Such pharyngeal gills are a unique feature of chordates. Huge in relation to the overall size of the body, the lancelet's pharynx is pierced by approximately 90 pairs of gill slits. Tunicates also have a similar pharyngeal chamber. Lampreys are characterized by 7 pairs of gill sacs, while hagfishes have from 6 to 14 pairs. The typical number of gill slits in fish is 5, although some primitive sharks have 7. In most sharks, another one, the anterior one, is modified in the squirter and noticeably separated from the rest. Ganoid fish also have a squirter. In ancient times, one of the groups of primitive freshwater fish (lobe-finned fish) acquired lungs as additional respiratory organs. They arise in the embryo as a protrusion of the abdominal wall of the pharynx, which takes on a tubular shape, grows backward and bifurcates, turning into two hollow sacs. Later they move to the dorsal wall of the body cavity and are surrounded by a special membrane, the pleura. The lungs lie below the epithelial lining of this wall (as opposed to the swim bladder, which is located above it) and receive blood from the pulmonary artery, which arises from the sixth branchial arterial arch. The swim bladder developed in the ancestors of modern bony fish. It arose as an unpaired protrusion of the upper wall of the pharynx and was eventually located along the entire body cavity above the lining of its dorsal wall, but below the kidneys (mesonephros). The swim bladder is supplied with blood not through the pulmonary artery, but through the celiac artery; The exception is mud fish (amiya). The listed differences between the lungs and the swim bladder indicate that they arose independently of each other and are non-homologous structures. However, the swim bladder is sometimes used as an additional organ of air respiration, especially in ganoids (mud fish, armored pikes and sturgeons). In African polypterus (Polypterus), the swim bladder is double, abdominal, necessary for breathing along with the gills and is served by the pulmonary arteries, i.e. is essentially lightweight. Cartilaginous fish have neither lungs nor swim bladders. The tube leading from the ventral side of the pharynx to the lungs is retained in adult animals as the trachea. In lungfish and amphibians this is a short channel with soft walls, and in reptiles, birds and mammals it is a hard tube with cartilaginous rings in the walls that prevent it from collapsing. The mammalian vocal chamber, the larynx, develops at the back of the pharynx at the entrance to the trachea and esophagus. In birds, the source of sounds produced is the additional lower larynx, located deep in the chest, where the trachea branches into two bronchi leading to the lungs. Thus, the vocal organs in birds and mammals are not homologous. Amphibian larvae living in water develop 3 pairs of external gills of ectodermal origin, not entirely homologous to the internal gills of fish. The larvae of African and South American lungfish are equipped with 4 pairs of external gills, while the polyfin larva has only one. Amphibians at different stages of their lives can breathe through moist skin, external gills, internal gills and lungs. Frogs and salamanders lacking a thorax, i.e. not capable of costal respiratory movements, they push air into the lungs, as if swallowing it, and exhale by contracting the muscles of the abdominal wall. Turtles breathe in a similar way due to the immobility of their shell, but other reptiles, as well as birds and mammals, ventilate their lungs by rhythmically expanding and contracting the chest. In birds, the lungs are directly connected to the chest. In addition, many air sacs extend from them, which are located between the internal organs and even in hollow bones. In mammals, the lungs are suspended freely in the chest cavity and fill as pressure drops in them. This cavity is separated from the abdominal cavity by a unique flat muscle, the diaphragm, which in a relaxed state forms a dome directed towards the head. Contracting during inhalation, it flattens, thereby enlarging the chest cavity and creating the pressure difference necessary for inhalation.
EXCRETORY SYSTEM
The excretory system removes metabolic waste from the body. Excretion products can be undigested food, sweat, carbon dioxide, bile (from the liver) or urine produced in the kidneys. Here only the kidneys and functionally related structures will be considered, i.e. specialized excretory organs of vertebrates.
Invertebrates. Excretion in protozoa is ensured by contractile vacuoles. In flatworms and some other invertebrates, primitive nephridia, or protonephridia, consisting of large “flame” cells and associated tubules, are used for this purpose. “Flame” cells function simultaneously as a filter and as a “motor” that ensures the flow of liquid excrement through the excretory system: metabolic waste and water enter them from the surrounding tissues, and they drive the resulting fluid into the tubules and further along the ducts to the excretory pores. In the recess of each “flame” cell there is a bunch of cilia (“flickering flame”), the beating of which drives liquid excrement through the excretory tubes from the body. In annelids, the excretory system is represented by nephridia of another type - the so-called. metanephridia. These are paired, metamerically located tubules, usually long and convoluted; one end of each tubule opens with a ciliated funnel into the coelomic cavity of the previous body segment, and the other - outward. The beating of the cilia creates a flow of fluid through the tubule, and as it moves, urine is formed. The excretory system of terrestrial invertebrates is structured differently. Their liquid excretory products exit through the Malpighian vessels into the hindgut, where water is absorbed; dehydrated excreta is expelled through the anus. This system allows you to reduce water loss by the body.
Vertebrates. In vertebrates, three types of kidneys appear successively: pronephros, mesonephros, and metanephros. The pronephros develops in the early embryo in the form of a cluster of a few tubes - nephrons (renal tubules) - along the anterior-superior part of the inner wall of the body cavity. From these, urine enters the primary ureter, called the pronephric, or Wolffian, canal. In all vertebrates, except hagfish, the pronephros functions only temporarily. Following this, similar but more complex tubes of the mesonephros are formed, which in fish and amphibians becomes a functional kidney. At the same time, the Wolffian canal is still used to excrete urine into the external environment or into the cloaca. In reptiles, birds and mammals, the third type of kidney, or metanephros, develops behind the mesonephros. It is even more complex histologically, works more efficiently and forms its own excretory channel, the secondary ureter. The Wolffian canal is preserved in males for the removal of sperm, but degenerates in females. Some reptiles (such as snakes and crocodiles) and birds do not have a bladder, and their ureters open directly into the cloaca. In mammals, they lead to the bladder, from which urine is excreted through the unpaired duct - the urethra. All animals, with the exception of oviparous animals, lack a cloaca.
Mesonephros of fish are long ribbons running along the dorsal side of the body cavity between the swim bladder and the bases of the ribs. In amphibians they are more compact and attached to the body wall by the mesentery. In snakes, the kidneys are very elongated and divided into lobules. In birds they are densely packed in paired cavities of the pelvic bones. In mammals they are bean-shaped or lobed. The kidneys of all gnathostomes, except mammals, are supplied with blood flowing through both arteries and veins; the latter form gate systems there. The portal system is the second network of capillaries that receives blood on its way from the dorsal aorta to the heart. It is always located in glandular organs, such as the liver, adrenal glands or kidneys. In mammals, kidney function requires high blood pressure, and it enters them only from the arteries.
Vertebrates. If the lancelet's gonads, located segmentally on both sides of the body cavity, are devoid of ducts, then all higher vertebrates have reproductive ducts, often quite complexly arranged. In sharks, large paired gonads are located in front near the dorsal side of the body cavity. The eggs are also large and after fertilization or develop in special chambers of the oviducts, the so-called. uterus, or are deposited in water, covered with a dense protective shell. The embryonic stage takes quite a long time, and by the time of birth or hatching, sharks manage to reach quite large sizes. In bony fishes and amphibians, the ovaries are relatively large; in a typical case, many small, shell-less eggs are swept into the water, where fertilization occurs. Reptiles and birds lay large, shell-covered eggs. In female birds, the ovary and oviduct develop only on the left side of the body, but in males both testes are retained. Some snakes and lizards give birth to live young, but most reptiles lay eggs, almost always burying them in the ground. The excretion of reproductive products or the birth of young in most vertebrates occurs through the cloaca, but in typical bony fish and mammals a separate opening is used for this.
All tetrapods and some fish have a channel for the exit of sperm from the testes, i.e. The vas deferens serves as the Wolffian canal, i.e. primary ureter protonephros. In females of higher vertebrates, the same channels as in sharks continue to function as oviducts, although with significant changes. In all vertebrates, except mammals and bony fishes, they open into the cloaca separately. In evolutionarily advanced mammals, both oviducts are, to one degree or another, united and form an unpaired chamber for bearing the baby - the uterus. During the evolution of vertebrates, their gonads increasingly move towards the posterior end of the abdominal cavity. In many mammals, the testes migrate from it to a special sac, the scrotum.
ENDOCRINE GLANDS
Animal glands can be divided into two categories - with excretory ducts (exocrine) and without them. In the second case, the released products enter the blood. Such glands are called endocrine, or endocrine glands. Many exocrine glands are located in the skin and secrete their secretions onto its surface (sometimes there are practically no formed ducts here). These include, for example, mucous, sebaceous, poisonous, sweat, mammary glands, and the coccygeal gland of birds. Inside the body of vertebrates there are exocrine glands such as the salivary, pancreas, prostate, liver and gonads. Some glands, such as the pancreas, ovaries, and testes, function as both glands at the same time. Endocrine glands secrete hormones that, together with the nervous system, coordinate the work of different parts of the body. In humans, this category includes the pineal gland (epiphysis), pituitary gland, thyroid gland, parathyroid glands, thymus gland, secretin-producing cells of the duodenum, islets of Langerhans in the pancreas, adrenal glands, testes and ovaries. The pituitary gland has a dual origin. During its formation, a protrusion grows down from the base of the diencephalon, which meets the upward-directed outgrowth of the roof of the oral cavity and forms a single whole with it. The pituitary gland produces several hormones and is present in all vertebrates. In sharks this is a large lobular gland.
Thyroid and parathyroid glands. The bilobed thyroid gland develops from an outgrowth of the pharyngeal fundus and is present in all vertebrates, starting with fish. The intensity of metabolism and the level of heat production, the condition of the skin and its derivatives, as well as molting processes in those animals to which it is characteristic depend. The parathyroid glands also develop from the wall of the pharynx. Their number varies in different vertebrates from 2 to 6. In humans there are 4, immersed in the posterior surface of the thyroid gland. They are involved in the regulation of calcium metabolism in the body.
Thyroid and pancreas. The thymus gland also develops from the embryonic pharynx, and in lower vertebrates it is one of the cervical glands. In mammals, it moves to the front of the chest. Its size is relatively large in newborns and young animals, and gradually decreases in adults. It plays an important role in the body's immune defense. The pancreas contains two types of secretory cells: exocrine, which produce digestive enzymes, and endocrine, which secrete the hormone insulin. In cyclostomes, these cells exist separately. The pancreas first appears as a single organ in fish. The adrenal glands are dual in nature and consist of two tissues, each of which secretes its own hormones. Their internal (brain) part develops from the nervous tissue of the embryo and secretes adrenaline. In lower vertebrates it can be distributed along the upper wall of the body cavity, remaining separate. The outer layer (cortex) of the adrenal glands secretes corticosteroids. The gonads produce three important hormones: testosterone (in the testes), estrogens (in the ovaries and placenta) and progesterone (in the corpus luteum of the ovary). Testosterone and estrogens stimulate the development of secondary sexual characteristics, male and female, respectively. All female sex hormones together control the sexual cycle. However, in females the physiology of sex is under triple control of the pituitary gland, thyroid gland and gonads.
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With an increase in knowledge about the structure of animals, about their similarities and differences in various characteristics, the possibilities for comparative anatomy and morphology, which grew on its basis, as a science about the laws of the structure of animals, expanded.
The great joys of comparative anatomy and morphology and their application for the classification of animals were associated in the first third of the 19th century. with the names of Cuvier and Geoffroy Saint-Hilaire.
Georges Cuvier was born in 1769 into the poor family of a retired officer. His interest in zoology arose under the influence of reading Buffon's Natural History. His development as a biologist was facilitated by his friendship with the gifted naturalist K. Kielmeyer. Cuvier acquired brilliant knowledge in the field of zoology through self-education, mainly during his eight-year stay in Normandy as a home teacher. In 1795, at the invitation of Etienne Geoffroy Saint-Hilaire, he came to Paris and in the same year became a professor and member of the French Institute (Academy of Sciences). Cuvier was distinguished by his enormous ability to work. Among his most significant works are “Lectures on Comparative Anatomy” (1800-1805, in five volumes), “The Animal Kingdom” (1817, in four volumes), “Studies on Fossil Bones” (1812, in four volumes; 4th edition, in ten volumes), “Natural History of Fishes” (1828-1833, in nine volumes), “History of Natural Sciences” (posthumously, 1845, in five volumes, edited by Saint-Azha). "
Comparative anatomy, animal taxonomy and paleontology ^* the three areas in which Cuvier worked were internally interconnected in his work and had a common theoretical basis.
Cuvier developed an idea of the nature of the organism already in the 90s of the 18th century.
In the first lecture of a course on comparative anatomy (1790), referring to Kant (obviously referring to § 66 of the “Critique of Judgment”), Cuvier wrote: “The mode of existence of each part of a living body is moved by the totality of all other parts, whereas in inorganic bodies each part exists on its own."*
Later, having developed this idea into the principle of correlation of parts, Cuvier formulated it as follows: “Every organized being forms a whole, a single closed system, the parts of which correspond to each other and contribute, through mutual influence, to one final goal. Not one of these parts can change without the others changing, and, therefore, each of them, taken separately, indicates and determines all the others.”2 As an example, Cuvier referred to the structure of a predator. If the intestines of this animal are designed in such a way that they can only digest fresh meat, then it should have
the jaws are built accordingly; the latter, in turn, must be equipped with teeth suitable for capturing and cutting prey; there must be claws on its limbs to grab and tear apart the victim; the entire system of movement organs must be adapted for its pursuit and catching; sense organs - to notice it from afar, etc. The correlation of parts reaches the finest details. “Indeed,” writes Cuvier, “in order for the jaw to grasp, it needs a known shape of the articular head, a known relationship between the position of the resistance ® force and the fulcrum, a known volume of the temporal muscle, which requires a known area of the fossa in which it lies, and the well-known convexity of the zygomatic arch under which it passes; the zygomatic arch must also have a certain strength in order to provide support for the masticatory muscle.” However, there are cases where the relationship of the parts is not clear enough. For example, why do animals have cloven hoofs and horns on their foreheads? Cuvier could not answer this question. To do this, it was necessary to study the evolution of the corresponding species, and Cuvier did not recognize evolution. Cuvier used the idea of correlation both to explain the relationships between organisms in nature (flies cannot exist without swallows, and vice versa), and to build a “natural system” of animals. Unlike Linnaeus and other taxonomists, he widely used data from comparative anatomy for the purpose of classifying animals. He believed that zoology and comparative anatomy complement each other, comparative anatomy provides material for constructing a natural system of animals, and the creation of such a system is necessary for sequential comparison of their organs.
A comparison of parts of animals of different groups showed that there are parts that are found in all animals of a certain group, and parts that are different in different groups. For example, all animals have a spinal column, united on this basis into one general group - vertebrates, while among the representatives of this group the teeth have a different structure; there are vertebrates that have three main types of teeth - incisors, canines and molars (humans and many mammals), there are animals that lack incisors in the upper jaw (artiodactyls), have only molars (incomplete teeth), etc. The spine, in this For example, there is a “necessary”, “predominant” feature, and teeth are a “subordinate” feature. The degree of “subordination” of signs varies. The provision about different degrees of significance of features during systematization is called the principle of “subordination of features.” Cuvier borrowed it from the botanist Antoine Jussier and used it productively in zoology. Based on the compilation of a systematic group from the “predominant” characteristic, Cuvier further “descended” to the “subordinate” and “variable” characteristics and thereby brought the classification to lower divisions. However, Cuvier conducted research in the reverse order. Moreover, since within groups with the same way of life a very clear interconnection of parts is found, the principle of correlation clearly emerged.
Cuvier described, compared and classified the organs themselves according to their function, continuing the tradition of Aristotle (organs of movement, sensory organs, etc.). Consistent and rigorous study of organs of different
GEORGES CUVIER 1769-1832
species of animals in his Lectures of 1800 was a step forward in the development of comparative anatomy. Such a comparative anatomical study of organs using an unprecedentedly large amount of material served as the basis for Cuvier’s important innovative ideas. In his famous book - “The Animal Kingdom, Distributed According to Its Organization in order to serve as a Basis for the Natural History of Animals and an Introduction to Comparative Anatomy” (1817) - he already in this very title emphasized the connection between systematics and comparative anatomy.
In place of the old classification of Linnaeus and others, and also contrary to the idea of a “ladder of creatures,” Cuvier divided the entire animal kingdom into four “branches,” which he also called “principal forms” or “general planes.” Later, at the suggestion of his student “Blainville,” they began to be called “types.” The semantic content of this term in taxonomy is somewhat different from that in morphology.
Cuvier distinguished four “branches” (“types”) of the animal kingdom: “vertebrates,” “molluscs,” “articulates,” and “radiates.” He believed that these four “branches” are sharply demarcated in their structure, and there are no transitional forms between them.
Cuvier interpreted the “natural system” as a distribution in which beings of the same kind would be more closely related than those belonging to other genera; the genera of one and the same order are closer together than the genera of all other orders, and so on. He did not pose the question of what explains this relationship of forms. Perhaps he attributed this to the tasks of the distant future.
Cuvier did not limit himself to the study of living forms, but also turned to the fossil remains of extinct animals and became one of
the founders of paleontology. He examined the skeletal remains of a number of fossil vertebrates and determined their places in the system. Based on his principle of correlation, Cuvier was able, with brilliant insight, to establish the nature and size of the lost parts of the skeleton and to restore the skeleton and appearance of extinct mammals and reptiles to individual surviving parts of the skeleton. He boldly said: “Give me one bone and I will restore the animal.” His reconstructions of disappeared animals made a huge impression on his contemporaries. True, Cuvier had some mistakes along the way.
A study of the fossil remains of animals has shown that many of them belong to extinct species, now found nowhere on Earth. It also turned out that the layers of the earth's crust belonging to different geological periods contained the remains of various species of animals. This indicates that at different periods of the Earth’s history there was a change in faunas (for example, extinct “oviparous” vertebrates appeared much earlier than viviparous ones). The establishment of this fact allowed Cuvier to create a method for determining the age of a geological layer.
To explain these facts, Cuvier, who did not like hypotheses, resorted to the most unsuccessful hypothesis - the theory of catastrophes, according to which, as a result of short-term cataclysms (flood, earthquake, etc.), the entire fauna of a certain area of the earth's surface allegedly perished and was then inhabited by completely different ones. animals.
The colossal factual material on comparative anatomy and paleontology, compiled into a “natural” system, as well as Cuvier’s methods, served as an excellent basis for the further development of zoology and paleontology. And although he himself rejected any evolutionary ideas of his time, in fact the material he collected served to substantiate evolution.
Another outstanding French scientist, a contemporary of Cuvier-Etienne Geoffroy Saint-Hilaire, took a different theoretical position. The slogan of all his scientific activities were the words: “Nature created all creatures according to one plan, identical in principle, but endlessly varying in detail.”
Geoffroy was born in 1772. Among his teachers was the outstanding French crystallographer Ayuy (Hayuy), who had a great influence on him. In 1793, Buffon's former colleague, the zoologist Daubanton, persuaded Geoffroy to take the chair of vertebrate zoology to continue Buffon's work.
In 1818, the first, and in 1822, the second part of “Philosophy of Anatomy,” Saint-Hilaire’s main theoretical work, was published.
He called his concept of the unity of type the “theory of analogues.” Geoffroy used the term “analogs” (this word was borrowed from Aristotle) to designate parts of the body that were identical from a morphological point of view, i.e. homologous. The essence of Geoffroy's concept was as follows: animals are built according to one morphological type or plan, the homologous parts of which are preserved in different species of animals, regardless of the form and function of these parts. For example, a human hand, like a forelimb, is homologous to the front leg of a horse, the wing of a bird, etc. If you compare their anatomical structure, you can find homology of bones (bones of the shoulder, forearm and hand), muscles, blood vessels, nerves, etc. d. This idea, which was firmly established in science, was a bold innovation at that time due to the generality of its formulation and
ETTIENNE GEOFFROY SAINT-HILAIR
1772-1844
a clear distinction between homological similarity and similarity in function and form, which the predecessors of Geoffroy Saint-Hilaire were not yet clear enough to understand.
Geoffroy developed two principles: the principle of connexions and the principle of balancing organs.
The principle of connexions (interconnections) of parts or “materials” means that homologous parts are always located in the same way relative to adjacent parts. For example, the humerus lies above the ulna and radius, while these two are located next to each other, etc. This “law of place” was known to comparative anatomists of the older generation - Camper, Daubanton, Vic d'Azir and others, but not so generally and distinct form.
The principle of connections was understood more clearly than others by Goethe in his time, when in 1795 he built the “osteological type” of vertebrates. But Geoffrey was not aware of Goethe's work, and he developed this principle on his own. Geoffroy considered the principle of connexions as a “compass”, “Ariadne’s thread” in his studies of the unity of the morphological type of animals. He believed that “the organ will be changed, atrophied, destroyed rather than moved.” Finding the location of a given part was Geoffroy's main method of homologization. And to this day, after other homologation criteria have been found, the place occupied by the morphological “element” in the body system remains an important criterion for homologization.
Despite the errors and weaknesses of Etienne Geoffroy Saint-Hilaire's theory, it was a significant step forward in the development of the idea of homology,
and in connection with this, the ideas of morphological type, morphology in general. That is why the “theory of analogues” was useful for evolutionary teaching and the construction of a phylogenetic system of animals.
Chauffroy, like Goethe, borrowed the principle of balancing or “balancing the organs” from Aristotle. According to this principle, an organ achieves its full development only due to the underdevelopment of another organ from its system or adjacent to it. Thus, the increase in the length of the giraffe’s legs occurred, according to Chauffroy, due to a decrease in the size of the body. In our time, this principle retains its meaning in a more complex form (Om. Bertalanffy, 1949).
Vestigial organs and various developmental anomalies, which Chauffroy studied a lot (he was one of the founders of the science of deformities - teratology, in particular experimental), received a convincing explanation in the light of his theory.
In an effort to extend the idea of type unity to invertebrates, Geoffroy tried to prove that crayfish and insects are the same vertebrates, in which all internal organs are located inside the vertebrae. It is strange that he did not take into account the obvious violation of his own principle of connections.
Geoffroy believed that the diversity of animal forms with a common structure plan (“diversity in unity,” in the words of Leibniz, to whom Geoffroy Saint-Hilaire liked to refer) could be explained by the influence of the environment. He collected and discussed various facts related to both the field of individual development and evolution. He considered very significant the experiments of his friend Edwards (1824) with the delay of metamorphosis in tadpoles in the event of their prolonged stay under water.
In the article “On the degree of influence of the environment on the change in the forms of animals” (1833), Geoffroy wrote: “Every year we are present at a spectacle that is accessible not only to the spiritual, but also to the physical. eyes In our chapters there is a transformation and transition from the organic conditions of one class of animals to the conditions of another class. This is the case in Batrachia. Batrachia is at first like a fish - under the name of a tadpole, and then a reptile (amphibian according to modern nomenclature - Author) - under the name of a frog"
Comparing individual development with a systematic series of forms. Geoffroy sees a certain parallelism between them. The role of this idea in biology, which was developed before Geoffroy Saint-Hilaire by Kielmeyer and German natural philosophers, then by Geoffroy’s student E. Serre and especially by J.F. Meckel, who called this phenomenon the “law of parallelism,” will be discussed below. It is important to note here that Geoffroy expressed a wonderful idea in connection with this idea - the relationships between different species, the transitions between them are discovered when studying embryos.
Developing Buffon's ideas about the variability of animals and sympathizing with the ideas of Lamarck, Geoffroy tried to show the transformation of one species into another using paleontological data. He studied the fossil remains of large, reptile-like crocodilians (to which Cuvier classified them) and constructed a small series of four genera of the teleosaur family, linking modern crocodiles with their extinct ancestors. He confidently stated that “living animals come from
through a continuous chain of generations from extinct animals of the pre-diluvial period" *. Geoffroy was convinced of the transformation of organic forms. He began to defend this idea especially actively in the 30s.
In his penchant for broad scientific generalizations and for defending the idea of the unity of the organic world, Geoffroy was close to the German natural philosophers of his time.
From what has been said about the scientific views of Cuvier and Saint-Hilaire, the contradictions between their views and the differences in the methods of their work are quite clearly visible. This led to a clash at the famous debate in Paris in 1830.
The doctrine of type, in addition to Cuvier and Geoffroy and independently of them, was developed by W. Goethe and K. M. Baer.
The concept of morphological type was actually first formulated by Wolfgang Goethe. Goethe outlined his doctrine of morphological type in the article “First sketch of a general introduction to comparative anatomy, based on osteology” (1795) and in “Lectures” on the first three chapters of this sketch (1796). Both of these works were published only in 1820, after Geoffroy spoke with similar ideas. In his doctrine of morphological type, Goethe proceeded mainly from Buffon's idea of the variability of organic forms, which he set out in Natural History. Goethe developed it further and clearly illustrated it with the “osteological type” of mammals.
Goethe sought to theoretically substantiate the existence of morphology as a special biological discipline. The name “morphology” itself was proposed by Goethe. He characterized it as the science of “the formation and transformation of organic beings,” treating the form and structure of organisms as a dynamic process occurring over time. According to his ideas, a type is revealed in its countless “metamorphoses,” that is, in a multitude of real images that are, as it were, its variants, for which it serves as a “law”; a type is something constant in endless changes. Thus, in different species of mammals the skull includes the same bones. At the same time, in each species these bones have their own characteristics, and in each individual the same bone changes in a certain way in the process of individual development; it is always the same and at the same time different at different times.
Goethe figuratively called the type Proteus, the name of that mythical deity of the Greeks who easily changed his appearance, remaining himself. The introduction to the idea of the type of temporary element favorably distinguished Goethe's morphology from the similar morphology of Geoffroy, who thought more statically.
Baer approached the problem of type from the point of view of his specialty (see V scholium in “History of the Development of Animals,” vol. 1, 1828). By studying the embryos of different stages of development of various vertebrates, Baer discovered that in the earliest stages the embryos of even distant species are so similar that they are difficult to distinguish. In the process of development, specific characteristics are increasingly revealed in them - first of the class, then of the order, family, etc., and, ultimately, of the given individual. Based on embryonic development, Baer established four “basic types” of animals, which
which coincided with Cuvier’s four types, obtained on the basis of comparative anatomical data.
In the dispute between Cuvier and Geoffroy Saint-Hilaire, Baer was on Cuvier’s side, and Goethe was on Saint-Hilaire’s side,
Cuvier is rightly considered the founder of comparative anatomy, or, as they say today, comparative morphology. But Cuvier had predecessors in this field - in particular, Vic d'Azir. Cuvier's merit - and, moreover, not surpassed by anyone - lies in the fact that he broadly and generously expanded the base of arguments in defense of the doctrine of analogues, homologues and correlations, deepened the interpretation of the problems of morphology, superbly formulated its first “laws”... Georges Leopold Christian Dagobert Cuvier ( 1769–1832) was born in the small Alsatian town of Montbéliard. The boy was amazing with his early mental development. At the age of four he was already reading. Reading became Cuvier's favorite pastime, and then his passion. His favorite book was Buffon's Natural History. Cuvier constantly redrew and colored illustrations from it. At school he studied brilliantly. At the age of fifteen, Cuvier entered the Karolinska Academy in Stuttgart, where he chose the faculty of cameral sciences. Here he studied law, finance, hygiene and agriculture. But most of all he was drawn to the study of animals and plants. Almost all of his comrades were older than him. Among them there were several young people interested in biology. Cuvier organized a circle and called it an “academy.” Four years later, Cuvier graduated from the university and returned home. My parents were getting old and my father’s pension was barely enough to make ends meet. Cuvier learned that Count Erisi was looking for a home teacher for his son. Cuvier traveled to Normandy in 1788, on the eve of the French Revolution. There, in a secluded castle, he spent the most turbulent years in the history of France. The estate of Count Erisi was located on the seashore, and Cuvier for the first time saw alive sea animals, familiar to him from drawings. He dissected these animals and studied the internal structure of fish, crabs, soft-bodied fish, starfish, and worms. He was amazed to find that in the so-called lower forms, in which the scientists of his time assumed a simple body structure, there was an intestine with glands, a heart with blood vessels, and nerve ganglia with nerve trunks extending from them. Cuvier penetrated with his scalpel into a new world in which no one had yet made accurate and thorough observations. He described the research results in detail in the journal Zoological Bulletin. When Count Erisi's son turned twenty in 1794, Cuvier's service ended and he again found himself at a crossroads. Parisian scientists invited Cuvier to work at the newly organized Museum of Natural History. In the spring of 1795, Cuvier arrived in Paris. He advanced very quickly and in the same year he occupied the department of animal anatomy at the University of Paris - Sorbonne. In 1796, Cuvier was appointed a member of the national institute, and in 1800 he took the chair of natural history at the College de France. In 1802 he took the chair of comparative anatomy at the Sorbonne. Cuvier's first scientific works were devoted to entomology. In Paris, studying the rich collections of museums, Cuvier gradually became convinced that the Linnaean system accepted in science did not fully correspond to reality. Linnaeus divided the animal world into 6 classes: mammals, birds, reptiles, fish, insects and worms. Cuvier proposed a different system. He believed that in the animal world there are four types of body structure, completely different from each other. Deep knowledge of animal anatomy allowed Cuvier to reconstruct the appearance of extinct creatures from their preserved bones. Cuvier became convinced that all the organs of an animal are closely connected with each other, that each organ is necessary for the life of the entire organism. Each animal is adapted to the environment in which it lives, finds food, hides from enemies, and takes care of its offspring. If this animal is a herbivore, its front teeth are adapted to pluck grass, and its molars are adapted to grind it. Massive teeth that grind grass require large and powerful jaws and corresponding chewing muscles. Therefore, such an animal must have a heavy, large head, and since it has neither sharp claws nor long fangs to fight off a predator, it fights off with its horns. To support the heavy head and horns, a strong neck and large cervical vertebrae with long processes to which muscles are attached are needed. To digest a large amount of low-nutrient grass, you need a voluminous stomach and a long intestine, and, therefore, you need a large belly, you need wide ribs. This is how the appearance of a herbivorous mammal emerges. “An organism,” said Cuvier, “is a coherent whole. Individual parts of it cannot be changed without causing changes in others.” Cuvier called this constant connection of organs with each other “the relationship between the parts of the organism.” The task of morphology is to reveal the patterns to which the structure of an organism is subject, and the method that allows us to establish the canons and norms of organization is a systematic comparison of the same organ (or the same system of organs) across all sections of the animal kingdom. What does this comparison give? It precisely establishes, firstly, the place occupied by a certain organ in the animal’s body, secondly, all the modifications experienced by this organ at various stages of the zoological ladder, and thirdly, the relationship between individual organs, on the one hand, and also by them and the body as a whole - on the other. It was this relationship that Cuvier qualified with the term “organic correlations” and formulated as follows: “Each organism forms a single closed whole, in which none of the parts can change without the others also changing.” “A change in one part of the body,” he says in another of his works, “affects the change in all the others.” You can give any number of examples illustrating the “law of correlation”. And it’s not surprising, says Cuvier: after all, the entire organization of animals rests on him. Take any large predator: the connection between individual parts of its body is striking in its obviousness. Keen hearing, keen vision, well-developed sense of smell, strong muscles of the limbs, allowing one to jump towards prey, retractable claws, agility and speed in movements, strong jaws, sharp teeth, a simple digestive tract, etc. - who does not know these “relatively developed" features of a lion, tiger, leopard or panther? And look at any bird: its entire organization constitutes a “single, closed whole,” and this unity in this case manifests itself as a kind of adaptation to life in the air, to flight. The wing, the muscles that move it, a highly developed ridge on the sternum, cavities in the bones, a peculiar structure of the lungs that form air sacs, a high tone of cardiac activity, a well-developed cerebellum that regulates the complex movements of the bird, etc. Try to change something anything in this complex of structural and functional features of the bird: any such change, says Cuvier, will inevitably affect to one degree or another, if not all, then many other features of the bird. In parallel with correlations of a morphological nature, there are physiological correlations. The structure of an organ is related to its functions. Morphology is not divorced from physiology. Everywhere in the body, along with the correlation, another pattern is observed. Cuvier qualifies it as a subordination of organs and a subordination of functions. The subordination of organs is associated with the subordination of the functions developed by these organs. However, both are equally related to the animal’s lifestyle. Everything here should be in some harmonious balance. Once this relative harmony is shaken, the further existence of an animal that has become a victim of a disturbed balance between its organization, functions and conditions of existence will be unthinkable. “During life, the organs are not just united,” writes Cuvier, “but they also influence each other and compete together in the name of a common goal. There is not a single function that does not require the help and participation of almost all other functions and does not feel to a greater or lesser extent the degree of their energy... It is obvious that proper harmony between mutually acting organs is a necessary condition for the existence of the animal to which they belong, and that if any of these functions are changed out of conformity with the changes in other functions of the organism, then it will not be able to exist.” So, familiarity with the structure and functions of several organs - and often just one organ - allows us to judge not only the structure, but also the way of life of the animal. And vice versa: knowing the conditions of existence of a particular animal, we can imagine its organization. However, Cuvier adds, it is not always possible to judge the organization of an animal on the basis of its lifestyle: how, in fact, can one connect the rumination of an animal with the presence of two hooves or horns? The extent to which Cuvier was imbued with the consciousness of the constant connectedness of the parts of an animal’s body can be seen from the following anecdote. One of his students wanted to joke with him. He dressed up in the skin of a wild sheep, entered Cuvier’s bedroom at night and, standing near his bed, shouted in a wild voice: “Cuvier, Cuvier, I will eat you!” The great naturalist woke up, stretched out his hand, felt the horns and, examining the hooves in the semi-darkness, calmly answered: “Hooves, horns - a herbivore; You can’t eat me!” Having created a new field of knowledge - comparative anatomy of animals - Cuvier paved new paths of research in biology. Thus, the triumph of evolutionary teaching was prepared.
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