ZOOLOGY OF CHORDATES: STUDY GUIDE - Zakharenko M.O. - 2015

CHAPTER 4. SUPERCLASS TETRAPODA. LOWER TERRESTRIAL VERTEBRATES

4.4. Class Reptiles, or Reptilia

Amniotes are entirely terrestrial vertebrates. They belong to the group of animals known as amniotes. Unlike anamniotes, these are the first evolutionarily animals to fully adapt to a Water-free lifestyle.

Anamniotes are primarily aquatic vertebrates that lack embryonic membranes and the embryonic Urinary Bladder. Their eggs develop in water, covered by a gelatinous envelope that maintains the egg's shape and ensures its survival in the aquatic environment. There is no outer shell, the yolk is moderate in size, and the water required for the embryo enters from the outside through permeable egg membranes. Metabolic waste products are eliminated via the same pathway. Anamniotes include Fishes and amphibians. External Fertilization is typical for most anamniotes, though internal fertilization occurs in some cartilaginous fishes and certain amphibians. The egg develops in water, the hatched larva leads an aquatic lifestyle, and only a few amphibians develop adaptations that allow egg development outside of water.

Amniotes are terrestrial vertebrates. Their eggs always possess a hard outer shell and embryonic membranes — the serosa, amnion, and allantois (Fig. 169).

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Fig. 169. Four stages of embryonic membrane development in birds:

1 - ectoderm, 2 - entoderm, 3 - mesoderm, 4 - gut cavity, 5 - extraembryonic cavity, 6 - amnion, 7 - amniotic cavity filled with Amniotic Fluid where the embryo floats, 8 - serosa, 9 - allantois, 10 - yolk sac

During early development, a circular fold begins to form around the embryo. As it enlarges, it envelops the embryo. Its edges fuse in such a way that the inner and outer layers become continuous. The outer layer of the membrane is called the serosa, and the inner layer is the amnion. The embryo thus becomes enclosed within the amniotic cavity, which accumulates a special amniotic fluid in which the embryo floats, much like an anamniote embryo floats in water. Thus, the amniotic fluid in the former case and water in the

latter protect the embryos from desiccation and various mechanical damages. However, an amniote embryo located within a small cavity cannot breathe like an anamniote embryo, nor can it excrete waste products directly into the external environment. Consequently, simultaneously with The formation of the amnion, the amniote embryo develops a specialized embryonic organ — the allantois, or embryonic urinary bladder. The latter arises as an outgrowth of the posterior part of the embryo's gut located between the amnion and the serosa, transforming into a large sac into which the embryo deposits its waste products.

Fertilization in these animals is internal. Based on their reproduction Methods, amniotes are divided into oviparous, ovoviviparous, and viviparous species. The shell membrane features tiny tubules that facilitate the passage of oxygen. These pores act as a filter, protecting the embryo from microbial infections and fungal spores. During Embryogenesis, a portion of the shell salts dissolves, enters the embryo, and is utilized in Skeleton formation. The embryo can develop within the egg exclusively on land or inside the maternal Organism. The larval stage is absent, and body development is not accompanied by metamorphosis.

In amniotes, the egg Cell is supersaturated with yolk, which serves as the embryo's energy resource. During development, the embryo is surrounded by amniotic fluid secreted by the Cells of the amnion. This fluid prevents desiccation and mechanical injury to the embryo. Within the amnion, a special embryonic urinary bladder — the allantois — is formed. It eliminates metabolic waste products into the extraembryonic cavity and facilitates gas exchange between the amnion and the serosal membrane. In higher mammals, the allantois participates in the Formation of the Placenta. These Structural Features of the egg and embryonic development allow living animals to break free from aquatic dependence and colonize all terrestrial habitats.

The adaptive features of amniotes for life on land are evident in their external and internal Structure as well as in their embryonic development. Fundamental changes occurred particularly in The structure of the eggs, which acquired egg and embryonic membranes. Unlike anamniote eggs, where Proteins serve as the primary energy source, in amniote eggs this function is performed by Lipids, which break down to produce carbon dioxide and water. The end product of Protein Catabolism is uric acid. It is characterized by low diffusibility and therefore causes no harm to the embryo. Furthermore, the allantois serves as the embryo's respiratory organ; its outer wall, rich in Blood Vessels, fuses with the serosa, allowing gas exchange between the blood in the allantoic capillaries and the external environment through the porous eggshell. Such a process is characteristic of lower amniotes (reptiles and birds). In higher amniotes (mammals), the allantois undergoes further modification and participates in the formation of the placenta, or afterbirth. The placenta is a specialized organ that establishes a connection between the embryo and the maternal body. Amniotes retain only the first pair of gill slits (in the form of the Middle ear), while all others disappear during the Cytology/cytology/16.html">Early stages of embryonic development. Thanks to these embryonic adaptations, amniotes were able to finally sever their ties with the aquatic environment and spread across all terrestrial areas, unlike semi-aquatic organisms.

Reptiles are the first class of true primary-terrestrial vertebrates. Characteristic Features of reptiles include:

1. Large, yolk- and protein-rich eggs covered with a tough parchment-like membrane.

2. Internal fertilization exclusively.

3. Embryonic development occurring in a gaseous environment with the formation of embryonic membranes — the amnion, serosa, and allantois.

4. Absence of a larval stage.

5. Newly hatched young differing from adults only in size.

6. Dry Skin almost entirely devoid of glands.

7. Cornification of the outer epidermal layers: the formation of epidermal scales and shields in the skin.

8. Pulmonary Respiration exclusively. The Development of air passages — the Trachea and Bronchi. Breathing is driven by chest movements.

9. A three-chambered Heart. From the ventricle, divided by an incomplete septum, three arterial trunks independently depart: two aortic arches and a pulmonary artery.

10. The carotid Arteries, which supply arterial blood to the HEAD, branch off exclusively from the right aortic arch.

11. The SYSTEMIC AND PULMONARY circulations are not completely separated, but the degree of their Separation is higher than that in amphibians.

12. Excretion and water balance are maintained by metanephric (pelvic) Kidneys.

13. The relative size of the Brain is increased, particularly due to the enlargement of the cerebral hemispheres and Cerebellum.

14. The skeleton is entirely ossified.

15. The Axial Skeleton (Vertebral Column) is divided into five regions.

16. An elongated neck and the first two specialized cervical vertebrae (atlas and axis) provide a high degree of head mobility.

17. The Skull possesses a single occipital condyle and well-developed dermal bones, temporal fossae, and surrounding bony arches.

18. Limbs are of the terrestrial type with intercarpal and intertarsal joints.

19. The Pectoral Girdle is connected to the axial skeleton via Ribs.

20. The Pelvic Girdle articulates with the transverse processes of two sacral vertebrae.

21. Body Temperature is variable and largely depends on the ambient temperature (poikilothermy).

22. The overall metabolic and physiological activity level is markedly higher than that of amphibians.

Structural features of reptiles. The body shape of reptiles is more diverse than that of amphibians (Fig. 170).

Fig. 170. Diversity of reptiles

This is related to their modes of locomotion. Serpentine reptiles (snakes, legless lizards) have lost their limbs, yet they successfully move through leaf litter, grassy vegetation, and tree canopies, as well as swim proficiently.

The body of reptiles is divided into the following regions (Fig. 171).

Fig. 171. External structure: lizard (A):

1 - head, 2 - cervical region, 3 - trunk region, 4 - caudal region, 5 - forelimbs and hindlimbs, 6 - tympanic membrane, 7 - eye, 8 - Mouth, 9 - nostrils of a lizard; (B): 1 - Tongue, 2 - nostrils, 3 - eye, 4 - ear opening, 5 - scales of an alligator (C) and a turtle (D): 1 - head, 3 - trunk,

4 - tail, 5 - hind limbs

Integument. The upper layers of the multilayered reptile epidermis undergo keratinization: the cells become filled with granules of the protein keratin, which displace the Cytoplasm and Nucleus. Beneath this dead layer lies the lower Malpighian layer, which consists of living, actively dividing epidermal cells (Fig. 172).

Fig. 172. Longitudinal section of lizard skin (after Büchi):

1 - epidermis; 2 - dermis (corium); 3 - stratum corneum; 4 - Malpighian layer; 5 - pigment cells; 6 - dermal ossifications.

The proliferation of the stratum corneum gives rise to scutes, scales, keratin granules, tubercles, spines, and claws. In some reptile species, bony plates develop beneath the epidermal scales within the dermal layer of the skin, known as the corium. In turtles, these plates fuse to form a bony carapace that is fused with the spine. Pigment cells are located in the Malpighian layer and the upper PARTS OF THE corium. The skin provides reliable protection against water evaporation, mechanical damage, and the penetration of pathogens. At the sameized time, it has lost its capacity for gas exchange, water evaporation, and The excretion of metabolic waste products. The skin fits tightly to the body. The keratinous covering is renewed through molting—the complete or partial shedding of the old keratin sheath and the formation of a new one. For many species, molting occurs several times a year.

Skeleton. THE VERTEBRAL COLUMN of reptiles is divided into five regions: cervical, thoracic, lumbar, sacral, and caudal (Figs. 173 - 175).

Fig. 173. Lizard skeleton:

1 - lower jaw; 2 - upper jaw; 3 - skull; 4 - cervical region; 5 - thoracic region; 6 - coccygeal region; 7 - coracoid; 8 - thoracic region; 9 - ribs; 10 - scapula; 11 - pectoral girdle; 12 - clavicle; 13 - Sternum; 14 - pelvic girdle; 15 - humerus; 16 - forearm; 17 - manus (hand); 18 - Femur; 19 - lower leg; 20 - FOOT

Fig. 174. Turtle skeleton:

1 - skull; 2 - cervical vertebrae; 3 - scapula; 4 - humerus; 5 - ulna; 6 - radius; 7 - thumb; 8 - middle finger; 9 - little finger; 10 - trunk vertebrae; 11 - plastron; 12 - pelvis; 13 - caudal vertebrae; 14 - Fibula; 15 - Tibia

Fig. 175. Snake skeleton: A: General view; B: Pelvic girdle:

1 - longitudinal bone; 2 - puboischiadic bone; 3 - femur; 4 - ungual phalanx

The evolution of a flexible neck and enhanced head mobility played a crucial role in foraging and orientation for reptiles. Head mobility is ensured by the presence of the first two cervical vertebrae—the atlas and the axis (epistropheus). The atlas resembles a bony ring that accommodates the odontoid process of the second cervical vertebra, the axis (Fig. 176).

Fig. 176. Monitor lizard vertebrae:

A - atlas; B - axis; C - thoracic vertebra; D - longitudinal section of a thoracic vertebra:

1 - odontoid process of the axis, 2 - vertebral centrum, 3 - neural arch, 4 - neural spine, 5 - spinal canal,

6 - prezygapophysis, 7 - postzygapophysis

Neck mobility depends on the number and STRUCTURE OF THE cervical vertebrae, which vary among different groups. Tuataras have amphicoelous (fish-like) vertebrae with remnants of the notochord between them. In crocodilians and most squamates, the vertebrae are procoelous (concave anteriorly), and only in a few primitive forms are they amphicoelous. Some of the cervical vertebrae bear short ribs. The complex Movements of the turtle's neck are made possible by a diversity of vertebra types: the posterior vertebrae are procoelous, the anterior ones are opisthocoelous (concave posteriorly), and the middle ones are amphicoelous. Long ribs articulate with the thoracic vertebrae, and their ventral ends attach to the sternum via cartilaginous sections, forming a closed rib cage (snakes lack a rib cage). The pectoral girdle attaches to the sternum. The lumbar vertebrae also bear ribs that do not reach the sternum. The pelvic girdle attaches to the sacral region, which consists of two vertebrae. The caudal region helps maintain balance during locomotion and sometimes serves as a propulsor (in aquatic snakes, crocodilians, and some aquatic lizards). In lizards capable of autotomy, the caudal vertebrae can fracture mid-body, as there are thin cartilaginous layers dividing the vertebral body into two parts. The total number of vertebrae in various reptile species can range from 50–80 (7–10 cervical, 16–25 thoracolumbar, 2 sacral, 15–40 caudal), up to 140 (in thick and short-bodied snakes), and 435 (in elongated snakes).

Paired limbs and their girdles. Unlike amphibians, the pectoral girdle of reptiles features a coracoid which, at its junction with the scapula, forms a glenoid cavity where the head of the humerus articulates (Fig. 177).

Fig. 177. Pectoral girdle and forelimb of the lizard Lacerta:

1 - clavicle; 2 - suprascapular Cartilage; 3 - scapula; 4 - coracoid; 5 - ribs; 6 - sternum; 7 - procoracoid cartilage; 8 - interclavicle;

9 - humerus; 10 - ulna; 11 - radius; 12 - carpus; 13 - metacarpus; 14 - Phalanges.

A thickened suprascapular cartilage fuses dorsally to the scapula, and a cartilaginous procoracoid attaches anteriorly to the coracoid. The coracoid and procoracoid on each side fuse with a median bony sternum, anchoring the forelimb girdle to the axial skeleton via the rib cage. A cruciform dermal bone, the interclavicle, attaches ventrally to the sternum. Paired dermal clavicles connect the anterior end of the interclavicle to the dorsal part of each scapula.

The pelvic girdle consists of two innominate bones; each is formed by the fusion of three pelvic bones: the ilium, ischium, and pubis. Together, they form the acetabulum, which articulates with the head of the femur to form the hip joint (Fig. 178).

Fig. 178. Pelvic girdle of a lizard:

1 - acetabulum for the femoral head; 2 - ilium; 3 - pubis; 4 - ischium; 5 - symphysis.

The iliac bones connect with the transverse processes of the sacral vertebrae. In all modern reptiles, the pelvis is closed: the right and left pubic and ischial bones join each other along the midline via a symphysis, which is a cartilaginous bridge. Paired limbs in various reptile species and groups differ depending on their specific modes of locomotion. However, they share the General structural plan of the paired limbs typical of terrestrial vertebrates.

The shape of the reptilian skull depends primarily on The Nature of their diet and methods of obtaining food. The skull features elongated jaws that form a long snout. This jaw shape allows them to tear flesh from large prey. Concurrently, the mass of the masticatory musculature increases. The skull consists almost entirely of bone (Fig. 179).

The occipital region is formed by four endochondral bones: the supraoccipital, the basioccipital, and two exoccipitals. They frame the foramen magnum, at the bottom of which lies a single occipital condyle formed by the basioccipital and the two exoccipitals. The dermal basisphenoid lies anterior to the basioccipital and forms the floor of the braincase. A small parasphenoid grows onto it anteriorly, along with paired vomers, flanked by the choanae. Three otic bones develop in the region of the auditory capsule: the prootic, which remains independent; the opisthotic, which fuses with the exoccipital; and the epiotic, which fuses with the supraoccipital. The olfactory region lacks bones and remains cartilaginous.

Fig. 179. SKULL OF A lizard:

A - dorsal view; B - ventral view; C - lateral view: 1 - foramen magnum; 2 - exoccipital; 3 - supraoccipital;

4 - basioccipital; 5 - occipital condyle; 6 - basisphenoid; 7 - Vomer; 8 - choana; 9 - parasphenoid;

10 - parietal; 11 - interparietal with the pineal foramen; 12 - frontal; 13 - nasal; 14 - premaxilla; 15 - Maxilla; 16 - prefrontal; 17 - lacrimal; 18 - palpebrals (supraorbitals); 19 - postfrontal or postorbital; 20 - squamosal; 21 - supratemporal; 22 - temporal; 23 - nostril; 24 - quadrate; 25 - palatine;

26 - pterygoid; 27 - epipterygoid (columella cranii); 28 - ectopterygoid (transverse bone); 29 - articular; 30 - dentary; 31 - surangular; 32 - coronoid.

The skull roof is formed by paired dermal bones: nasals, prefrontals, frontals, and postfrontals; followed by the parietals and the unpaired interparietal, the latter possessing a foramen for the parietal (pineal) organ. The sides of the skull are formed by dermal bones: paired premaxillae (which fuse in some species); paired maxillae; palpebrals; jugals; quadratojugals; and squamosals. The posterior part of the palatoquadrate cartilage gives rise to paired endochondral ossifications, the quadrate bones. The anterior part of the palatoquadrate cartilage is replaced by dermal bones that form the floor of the skull: paired palatines and pterygoids. Transverse bones connect the pterygoids to the maxillae, while in lizards and tuataras, epipterygoids (columellae) additionally link the pterygoids to the parietals (Fig. 180).

Fig. 180. Diagram of evolutionary transformations of the stegskull in reptiles: A - stegskull, characteristic with modifications in anapsids;

B - diapsid skull with two temporal fenestrae; C - synapsid skull with a single lateral temporal fenestra: 1 - upper temporal fenestra;

2 - lower temporal fenestra; 3 - single lateral temporal fenestra; 4 - postfrontal (postorbital) bone; 5 - squamosal bone; 6 - jugal bone;

7 - quadratojugal bone.

In crocodiles and certain other reptiles, the expansion of the palatal processes of the premaxillae and maxillae, as well as the palatine bones, leads to the formation of a secondary bony palate. This divides the Oral Cavity into an upper section—the nasopharyngeal passage—and a lower section—the oral cavity proper (Fig. 181).

Fig. 181. Formation of the secondary palate. Schematic paramedian section through the skull of a primitive reptile (a) and a mammal (b):

1 - primary choanae; 2 - secondary choanae; 3 - nasopharyngeal passage; 4 - primary hard palate; 5 - secondary hard palate; 6 - Nasal cavity; 7 - external naris.

The lower jaw is formed from Meckel's cartilage, which is replaced by an endochondral articular bone that articulates with the quadrate, along with a series of dermal bones: the dentary, angular, surangular, coronoid, and sometimes a few additional small bones. The dorsal element of the hyoid arch (hyomandibula) has transformed into the stapes, an auditory ossicle of the middle ear. The hyoid apparatus consists of a cartilaginous plate (homologue of the copula) and three pairs of horns, which are homologous to the hyoids and the remnants of the gill arches. The Evolution of the masticatory musculature was accompanied by the restructuring of the roof and, partially, the lateral walls of the primitively solid stegskull inherited from reptilian ancestors.

The musculature of reptiles has changed significantly compared to amphibians. The primitive metameric arrangement of Muscles is preserved only in the musculature connecting adjacent vertebrae and in the Muscles of the abdominal wall. A powerful masticatory musculature develops within the temporal fenestrae of the skull to drive the jaws. The appearance of a mobile neck region was accompanied by the development of cervical musculature, which is especially well-developed in species with elongated and flexible necks. Ribbon-like muscles participate in body flexion and limb movement. The rudiments of cutaneous (subcutaneous) musculature appear, altering THE POSITION OF epidermal scales; this musculature is particularly well-developed in snakes and legless lizards, aiding in locomotion by coordinating the movement of belly scutes. The topography of Internal Organs is shown in Figure 182.

Fig. 182. Internal anatomy of a lizard (male):

1 - heart; 2 - trachea; 3 - Lungs; 4 - Gallbladder; 5 - Liver; 6 - Stomach; 7 - Pancreas; 8 - Small Intestine; 9 - Large Intestine; 10 - kidneys; 11 - urinary bladder; 12 - cloacal opening; 13 - Testes; 14 - vasa deferentia

The Digestive System of reptiles begins with the oral cavity. Reptiles capture food using their jaws. Most species swallow their food whole. Turtles, which lack Teeth, shear plant matter using the sharp edges of their horny jaw sheaths. The structure of the snake jaw apparatus allows them to open their mouths wide and swallow prey that exceeds the diameter of the snake's body. Consequently, the temporal arches have disappeared in the skull of snakes, and the jaw apparatus has transformed into a hinge-lever system (Fig. 183).

Fig. 183. Skull of a rattlesnake with the jaw closed (top) and open (bottom):

1 - venom fang, 2 - maxilla, 3 - transverse bone, 4 - Palatine bone, 5 - squamosal bone, 6 - quadrate bone

The jaws are armed with numerous sharp teeth. These teeth fuse to the jaws and palatine bones (pleurodont and acrodont teeth); only in crocodilians and fossil mammal-like reptiles is the Base of the teeth anchored in specialized sockets known as alveoli (thecodont teeth). In modern reptiles, the teeth are homodont (uniform); specialized venom fangs develop only in certain snakes. Crocodiles and turtles are capable of tearing large chunks of meat from sizeable prey.

Salivary Glands are located within the oral cavity of reptiles; their mucous secretions moisten the food and facilitate swallowing. In venomous snakes and certain lizard species, some salivary glands have evolved into venom glands, the secretion of which flows through a duct or an external groove on the venom fangs. Some glands have acquired The ability to produce digestive Enzymes (though their concentration in reptilian saliva is minimal). A muscular tongue, which the animal can project a considerable distance, is situated on the floor of the oral cavity.

Lizards and snakes possess a bifurcated tongue that Functions as an Olfactory Organ and, in conjunction with Jacobson's organ, as a chemoreceptor. The club-ended tongue of chameleons can be rapidly ballistically projected to capture small, mobile prey such as insects. The Esophagus is well-developed. In snakes, it features strong musculature capable of forcing large prey down into The Stomach. Demarcated from the esophagus, the stomach possesses muscular walls. The intestine is longer than that of amphibians. A primitive cecum branches off at the junction between the small and large intestines; it is better developed in herbivores (such as the steppe tortoise). The intestine terminates in the cloaca. The pancreas lies within the first loop of the intestine. The large liver contains a gallbladder, whose duct opens into the intestine adjacent to the pancreas (Fig. 184).

Fig. 184. Digestive system of reptiles

Modern reptiles are predominantly carnivorous. Most feed on small terrestrial and aquatic animals, the populations of which in nature are abundant and relatively stable; they also consume small vertebrates including fish, amphibians, and murine rodents. Aquatic and semiaquatic turtles and snakes feed on aquatic invertebrates and fish. Sea snakes feed primarily on cephalopods. Crocodiles and large snakes prey on large animals. Terrestrial and aquatic turtles, as well as tropical agamas and iguanas, are herbivorous reptiles.

Respiratory organs and gas exchange. The embryo developing within a reptilian egg ontogenetically corresponds to the larval stage of amphibians, respiring via the blood capillaries of the yolk sac and, later, the allantois. The skin of reptiles is covered with horny formations and does not participate in respiration. The primary respiratory organs after hatching are the paired lungs; in snakes, the right lung is larger (whereas in amphibians, it is the left). The lungs have a sac-like structure, but their internal architecture is significantly more complex than that of amphibians (Fig. 185).

Lung ventilation is driven by the action of the rib cage via intercostal and abdominal musculature. This novel mode of breathing is accompanied by a remodeling of the respiratory tracts: a non-collapsing breathing tube—the trachea—develops, its walls supported by elastic cartilaginous rings. The entrance to the trachea from the laryngeal chamber into the oral cavity opens via the glottis. At its posterior end, the trachea divides into two bronchi that enter the lungs and branch further into smaller bronchioles.

Fig. 185. Lungs of reptiles:

A - amphisbaenians (cross-section); B - anacondas (dorsal view); C - tuataras (cross-section); D - monitor lizard (cross-section); E - alligator (cross-section);

F - chameleon (ventral view; air sac-like appendages)

Circulatory system and Blood Circulation. Like amphibians, reptiles have two circulatory loops (Fig. 186).

Fig. 186. CIRCULATORY SYSTEM OF reptiles

The Heart is three-chambered. The atrium is divided by a complete septum; each atrium opens into the ventricle through a separate aperture equipped with a valve made of semilunar folds. The ventricle features an incomplete septum that divides it into two chambers (Fig. 187).

Fig. 187. Structure of the reptilian heart

Mixed blood enters the left aortic arch. Consequently, mixed blood flows through the dorsal aorta, with a predominance of arterial blood (Fig. 188).

Fig. 188. Diagram of the circulatory system of a lizard:

A - Arterial System; B - Venous system (vessels with arterial blood are shown in white, with mixed blood in dashed lines, and with venous blood in black): 1 - right atrium, 2 - left atrium, 3 - ventricle, 4 - pulmonary artery, 5 - pulmonary vein, 6 - right aortic arch, 7 - left aortic arch, 8 - dorsal aorta, 9 - iliac artery, 10 - caudal artery, 11 - carotid artery, 12 - carotid duct, 13 - Subclavian Artery, 14 - jugular Veins (a - internal, b - external), 15 - subclavian vein, 16 - anterior vena cava (a - right, b - left), 17 - sinus venosus, 18 - renal portal vein, 19 - caudal vein, 20 - pelvic vein, 21 - abdominal vein, 22 - HEPATIC PORTAL VEIN, 23 - renal vein, 24 - posterior vena cava, 25 - hepatic vein, 26 - lung, 27 - Kidney, 28 - liver, 29 - intestine, 30 — stomach

The venous system of reptiles is similar to that of amphibians. In the pelvic region, the caudal vein divides into two iliac or pelvic veins, which receive blood from the hind limbs. The iliac veins branch off into the renal portal veins and merge to form the abdominal vein. The abdominal vein, together with the hepatic portal vein carrying blood from the intestine, breaks up into capillaries within the liver. The liver detoxifies protein breakdown products, initiates the synthesis of excretory products, stores Glycogen reserves, and performs certain hematopoieic processes. The capillaries of the hepatic portal system converge into hepatic veins, which empty into the posterior vena cava. Blood from the head is carried by paired jugular veins. They join with the subclavian veins to form the right and left anterior vena cavae, which empty into the right atrium. The pulmonary vein, formed by the merger of the right and left Pulmonary veins (carrying arterial blood), enters the left atrium.

Excretory organs. Among the adaptations that enabled reptiles to transition to a terrestrial lifestyle, the replacement of the mesonephric (trunk) kidney (found in cyclostomes, fish, and amphibians) with the metanephric (pelvic) kidney is particularly significant. In reptiles, these appear as two compact bodies located near the dorsal wall of the pelvic region (Fig. 189).

Fig. 189. Urogenital System of a female (A) and a male (B) reptile:

1 - kidney; 2 - urinary bladder; 3 - urinary opening; 4 - Ovary; 5 - oviduct; 6 - oviduct funnel; 7 - genital opening (copulatory organ),

8 - cloacal cavity, 9 - rectum

During embryonic development, a pair of independent Ureters detaches from the posterior part of the Wolffian ducts (mesonephric ducts). They empty into the cloaca from the dorsal side.

Reproductive organs and reproduction. The Gonads lie within the body cavity on either side of the spine. Testes are paired oval bodies connected to vas deferens. The right and left vas deferens open into their respective ureters. Males of all reptiles, except the tuatara, possess specialized copulatory organs; in crocodilians and

turtles, these are unpaired, whereas in lizards and snakes, they are paired outgrowths of the posterior cloacal wall that evert during fertilization. The paired Ovaries appear as granular oval bodies. The Müllerian ducts extend from them, beginning with ciliated funnels located near the ovaries and ending with openings in the cloaca. Fertilization occurs in the upper section of the oviduct. Secretions from the glands in the middle section of the oviduct form an egg-white (protein) layer around the egg cell (yolk). This layer is poorly developed in snakes and lizards, but well developed in turtles and crocodilians. The outer egg membranes are formed from secretions produced by the cells lining the lower part of the oviduct (Uterus) (Fig. 190).

Fig. 190. Egg formation in reptiles

Most reptiles bury their laid eggs in the soil in well-warmed locations; some species deposit eggs in piles of plant debris or under rotting tree stumps, utilizing the heat generated by decomposition. Certain crocodiles dig pits and cover their eggs with plant residues, with females staying near the nest to guard the clutch. Some small lizards (such as monitor lizards) also guard their clutches. Female pythons coil their bodies around the clutch of eggs, guarding and incubating them. In such a "nest," the temperature is 6–12 ^oC higher than in the surrounding environment. In crocodilians, the female uncovers the clutch after the young hatch, facilitating their emergence to the surface; females of certain species also protect their offspring during the early stages of independent life. Ovoviviparity or, rarely, viviparity is characteristic of a small number of modern squamates (e.g., the common viper, smooth snake).

Nervous system, Sense Organs, and nervous activity. The reptilian brain differs from that of amphibians, notably by having a larger Forebrain; its enlargement is associated with the Development of the cerebral hemisphere vault and the expansion of the corpus striatum, which constitutes the bulk of the forebrain mass (Fig. 191).

Fig. 191. Brain structure of a reptile: 1 - forebrain; 2 - Diencephalon; 3 - Midbrain; 4 - cerebellum; 5 - Medulla Oblongata; 6 - olfactory lobe

The cerebral vault clearly displays the primary Pallium, or archipallium, which occupies a large portion of the hemisphere roof, as well as the primordium of the neopallium. The olfactory center in reptiles is more complex and differentiated compared to amphibians. The diencephalon is covered dorsally by the hemispheres of the forebrain. Its roof contains the Pineal Gland (an endocrine gland) and the parietal organ, which is capable of detecting light signals. The visual cortex of the midbrain is more developed than in amphibians. The reptilian cerebellum is large, corresponding to a significant increase in the complexity and intensity of their locomotion. The medulla oblongata forms a vertical tract characteristic of all amniotes, coordinating fundamental vegetative functions (respiration, circulation, Digestion, etc.). There are 11 pairs of Cranial Nerves originating from the brain. The visual organ is adapted for functioning in an aerial environment (Fig. 192).

Fig. 192. Reptile eye (longitudinal section of a snake's eye):

1 - iris muscles; 2 - anterior wall of the lens; 3 - cornea; 4 - ciliary body with muscles; 5 - infraorbital scale; 6 - sclera; 7 - supraorbital scale; 8 - fused eyelids; 9 - suspensory ligament of the lens.

The eye is protected by external eyelids and a nictitating membrane. Snakes and certain lizards (geckos, skinks, and some legless lizards) have fused eyelids that form a transparent protective covering. In nocturnal species, the eyes are enlarged and feature a vertical pupil. Lacrimal glands protect the eye from drying out. Visual accommodation is achieved by shifting the lens and altering its curvature using striated ciliary muscles.

The reptilian Organ of Hearing is similar to that of amphibians. It consists of a middle ear with a tympanic membrane and a single auditory ossicle—the stapes—which transmits vibrations from the membrane to the round window separating the Inner ear cavity. Within the inner ear, the cochlea is differentiated, serving as the apparatus for analyzing and encoding acoustic signals (Fig. 193).

Fig. 193. The auditory organ of reptiles

Chemoreceptors play a crucial role in the orientation and communication of reptiles. The olfactory organs open externally via paired nostrils and into the oral cavity through slit-like choanae (Fig. 194).

Fig. 194. The olfactory organs of reptiles

Many lizards rely on their SENSE OF SMELL to locate food, digging it out from sand at depths of up to 6–8 cm. Monitor lizards, racers, and vipers can use scent to distinguish between members of their own and other species. Turtles, lizards, and crocodiles possess specialized scent glands, the secretions of which are used to mark their established territories.

ORIGIN AND EVOLUTION of reptiles. The late Paleozoic era created environmental conditions highly favorable for the development of terrestrial animals, particularly reptiles. The divergence of reptiles from their amphibian ancestors began earlier, possibly in the middle Carboniferous, when forms adapted to terrestrial life branched off from primitive embolomerous stegocephalians (anthracosaurs), although they still maintained close ties to humid biotopes and water bodies. Feeding on small aquatic and terrestrial invertebrates, they were agile and possessed a slightly larger brain. Keratinization of the integument may have already begun in these forms. In the middle Carboniferous, a new branch—the Seymouriomorpha—arose from similar ancestors. Their fossil remains have been discovered in Upper Carboniferous to Lower Permian deposits. They occupy an intermediate evolutionary position between amphibians and reptiles; some paleontologists classify them as amphibians. Their vertebrae provided significant flexibility and strength to the spine, and the first two cervical vertebrae began to transform into the atlas and axis. For terrestrial animals, this offered considerable advantages in sensory orientation, catching mobile prey, and defending against predators. The Skeleton of the limbs and their girdles became fully ossified. The skull possessed an occipital process. The exact timing of The Emergence of reproduction and embryonic development in an aerial environment remains unclear, though it is believed to have occurred during the Carboniferous with the ESTABLISHMENT OF THE Cotylosauria. The skull roof was solid and composed of bones characteristic of modern reptiles. The atlas and axis were formed, and in some forms, the limbs elevated the body off the ground. Both small, lizard-like forms that fed on various vertebrates and large, herbivorous ones (up to 3 meters in length) existed.

Some cotylosaurs adopted a semi-aquatic lifestyle, while others likely became fully terrestrial inhabitants. The warm, humid climate of the Carboniferous favored amphibians. By the late Carboniferous and early Permian, intense mountain-building processes (the Hercynian orogeny, uplifting the Urals, Carpathians, Caucasus, Asia, and America) led to the fragmentation of relief, enhanced zonal contrasts (cooling at higher latitudes), and a reduction in the area of humid biotopes. This drove the establishment and diversification of terrestrial vertebrates. Cotylosaurs reached their evolutionary peak in the Permian and went extinct by the middle Triassic, likely due to competition with various progressive reptilian groups that had branched off from them. Turtles (Chelonia) also diverged from cotylosaurs during the Permian period.

In the Upper Carboniferous, two subclasses of reptiles emerged from cotylosaurs and transitioned to an aquatic lifestyle. Among these was the order Mesosauria, which comprised small, crocodile-like animals with long, slender teeth. Inhabiting water bodies and feeding presumably on fish, they were nonetheless capable of coming ashore. The order Ichthyosauria appeared in the Triassic, with fossil remains persisting into Upper Cretaceous deposits. In the seas of the Mesozoic era, they filled the ecological niche occupied today by cetaceans, morphologically resembling fish or dolphins (possessing a spindle-shaped body, elongated jaws armed with numerous teeth, a large two-lobed tail, and paired limbs shaped like flippers). Their body length ranged from 1 to 14 meters. They fed on aquatic invertebrates and fish, with larger ichthyosaurs preying upon smaller ones.

During the Permian, a major branch of diapsid reptiles—characterized by the development of two temporal fenestrae in the skull—diverged from cotylosaurs. This group later split into two subclasses: Lepidosauria and Archosauria. The Rhynchocephalia (tuataras and their relatives) also arose at this time, featuring large temporal fenestrae, a small beak at the tip of the upper jaws, and hook-like processes on the ribs. Although rhynchocephalians died out by the end of the Jurassic, a single surviving species—the New Zealand tuatara—has persisted to the present day. Squamates (lizards) became abundant and diverse during the Cretaceous, and snakes evolved from lizard ancestors toward the end of this period.

The heyday of squamates occurred in the Cenozoic era, though they still comprise the majority of modern reptiles. The subclass Archosauria was the most diverse in form and ecological specialization during the Mesozoic era, colonizing land, water, and eventually taking to the air. The ancestral group of archosaurs was the Thecodontia (or pseudosuchians), which diverged from eosuchians, likely in the Late Permian, and reached their zenith in the Triassic. They resembled lizards ranging from 15 cm to 3–5 m in length. Most led a terrestrial lifestyle and had hind limbs longer than their forelimbs. Another Lineage of thecodonts transitioned to a semi-aquatic lifestyle, ultimately giving rise to the Crocodilia at the end of the Triassic.

By the middle Triassic, flying reptiles, or Pterosauria, had evolved from thecodont ancestors. By the early Jurassic, specialized forms had emerged, ranging in size from a sparrow to giants with wingspans of 7–8 meters. They shared several morphological similarities with birds, including a wide sternum with a keel, pneumatic (hollow) bones, fused skull bones, and large eyes. Some species retained teeth, while in others these were reduced, with the jaws encased in a horny sheath (beak).

In the Upper Triassic, two distinct groups diverged from predatory pseudosuchians that moved primarily on their hind limbs: the Saurischia and the

- Ornithischia dinosaurs. These groups developed in parallel, and throughout the Jurassic and Cretaceous periods, they achieved an extraordinary diversity of species ranging in size from rabbits to massive titans weighing 30–50 tons. They inhabited terrestrial environments and the coastal zones of water bodies before going extinct at the end of the Cretaceous. The majority of saurischians were large carnivorous predators (reaching lengths of 10–15 meters) whose jaws were armed with formidable teeth and whose hind digits bore strong claws. A subgroup of saurischians shifted to herbivory and quadrupedal locomotion, including the largest land reptiles known—the sauropods, such as *Diplodocus*. Possessing long tails, flexible necks, and small heads, these animals reached body lengths of up to 30 meters and masses of 20–25 tons.

Ornithischian dinosaurs were predominantly herbivorous. Some moved on their hind legs with shortened forelimbs, while others included giants reaching lengths of 10–15 meters, such as *Iguanodon*.

Another major reptilian branch is the subclass Theromorpha, or Synapsida. Diverging from primitive Carboniferous cotylosaurs, they inhabited humid biotopes and retained numerous amphibian-like traits (such as glandular skin and limb structure). During the Upper Carboniferous and Permian, forms emerged that were grouped into the order Pelycosauria. They possessed amphicoelous vertebrae, a skull with a single temporal opening and a single occipital condyle, teeth on the palatal bones, and large abdominal ribs. Externally, they resembled lizards up to 1 meter in length, though isolated species reached 3–4 meters. They became extinct by the end of the Permian, having earlier given rise to the mammal-like reptiles, the Therapsida.

Thus, during the adaptive radiation of the late Permian and early Triassic, a diverse reptilian fauna comprising 13–15 orders flourished. Throughout the Mesozoic (approximately 150 million years ago), they held a dominant position in both aquatic and terrestrial biotopes.

At the end of the Cretaceous, a massive new mountain-building cycle (the Alpine orogeny) began on Earth, accompanied by profound landscape transformations, the redistribution of seas and landmasses, a general aridization of the climate, and an increase in seasonal and zonal climatic contrasts. Simultaneously, vegetation underwent a major shift: the dominance of cycads and conifers was replaced by angiosperms, whose fruits and seeds offered high nutritional value. These dramatic changes could not fail to impact the animal

kingdom, especially since two new classes of warm-blooded vertebrates—birds and mammals—had already evolved by this time. The surviving reptiles were simply unable to adapt to these altered living conditions, driven further toward extinction by intense competition with smaller, highly active birds and mammals. Representatives of these classes possessed physiological thermoregulation, high metabolic rates, and complex behavioral repertoires, which propelled their population growth and ecological importance. They rapidly and effectively adapted to changing landscapes, colonized new habitats with unprecedented speed, and intensively exploited novel food sources.

The modern Cenozoic era then commenced, with birds and mammals claiming dominance, while the surviving reptiles were restricted to small, agile squamates (lizards and snakes), well-protected turtles, and a modest group of aquatic archosaurs—the crocodilians.

Significance of Reptiles. Reptiles play a vital role in nature and human life, serving as integral components of most terrestrial biocoenoses. They provide a vital food source for numerous fur-bearing animals, such as polecats and foxes. In regions like South America, Asia, and Africa, local populations frequently keep non-venomous snakes instead of cats to control small rodents. In parts of Central Asia, land tortoises can damage melon fields, pistachio groves, and other agricultural crops. In certain fish-farming operations, dice snakes cause significant economic harm by preying on juvenile fish. Furthermore, land tortoises, certain lizards, and snakes act as vectors for ticks. In tropical and subtropical countries, venomous reptiles—such as rattlesnakes, cobras, vipers, and sea snakes—represent a serious hazard to humans. Historically, prior to the development of antivenoms, mortality rates from cobra bites reached 20–30%, and from vipers, 5–6%. Antivenoms are produced by gradually increasing the concentration of venom injected into horses, after which blood is drawn to manufacture the serum. Monovalent antivenoms (targeting a specific snake species in a given region) and polyvalent antivenoms (effective against the venom of multiple species) are both utilized. Due to the sharp decline in venomous snake populations, specialized serpentariums have been established. One of the earliest and largest facilities is a breeding center in Brazil near São Paulo, where scientists produce various antivenoms that save thousands of lives annually. Similar serpentariums operate in India, Indonesia, and other nations. Snake venoms are also utilized as a basis for pharmaceutical preparations (such as lachesin and viprasid), which are employed in the Treatment of rheumatism, radiculitis, vascular spasms, and Bronchial Asthma. The meat of large lizards and, especially, tortoises is consumed as food; for instance, giant tortoises and spur-thieved tortoises were nearly hunted to extinction during the 18th and 19th centuries. The keratinous shell of the hawksbill turtle has been widely used to manufacture boxes, combs, and other trinkets. The skin of large lizards, snakes, and particularly crocodiles has long been prized in The production of footwear and leather goods. The population decline of many reptile species has necessitated the creation of comprehensive conservation and sustainable management programs within specialized breeding facilities.

REPTILE Classification

Class REPTILIA

Subclass ANAPSIDA

Order TESTUDINES

Subclass LEPIDOSAURIA

Order RHYNCHOCEPHALIANS - RHYNCHOCEPHALIA

Order SQUAMATES - SQUAMATA

Suborder CHAMELEONS - CHAMAELEONIDAE

Suborder LIZARDS - SAURIA

Suborder WORM LIZARDS - AMPHISBAENIA

Suborder SNAKES - SERPENTES

Subclass ARCHOSAURS - ARCHOSAURIA

Order CROCODILIANS — CROCODILIA