ZOOLOGY STUDY GUIDE - Ye. O. Nevedomska - 2013

LECTURE 16. CLASS REPTILIA, OR REPTILES (Reptilia)

General characteristics of the Class Reptilia, or Reptiles (Reptilia)

Modern reptiles are distributed mainly in warm and temperate zones; some species have secondarily adapted to an aquatic lifestyle. The modern fauna includes about 7000 species of reptiles.

Herpetology (from Greek gerpeton — creeping things, logos — study) is the branch of zoology that studies reptiles.

The overall level of Organization in reptiles is significantly higher compared to amphibians; however, their body Temperature is variable and depends on the ambient temperature (poikilothermy).

Evolutionary advances of reptiles:

Advanced development of The Nervous System — The Emergence of the rudiment of the Cerebral Cortex. This led to The Development of adaptive behavior through The formation of conditioned Reflexes.

Formation of the metanephros (secondary Kidney), which ensures Water reabsorption in the renal tubules and a high concentration of Metabolic waste products in the urine.

The appearance of faveolar Lungs, the surface area of which significantly exceeds the internal surface area of the sac-like lungs of their ancestors. Their respiration is exclusively pulmonary.

The formation of a Diaphragm that separates the Abdominal cavity from the thoracic cavity and, together with the rib cage, provides aspiration-type breathing.

A completely ossified Skeleton and its progressive transformation: elongation of the neck, specialization of the first two cervical vertebrae (atlas and axis) ensuring high mobility of the HEAD; formation of the rib cage; terrestrial-type limbs, Pectoral Girdle

connected to the Axial Skeleton via the Ribs, and the Pelvic Girdle connected to the transverse processes of two sacral vertebrae.

Development of an incomplete septum in the ventricle of the three-chambered Heart, which ensures that the Brain and forelimbs are supplied exclusively with arterial Blood.

The emergence of embryonic membranes that ensure the Development of the embryo in terrestrial conditions. Large eggs rich in albumen and yolk. There is no larval stage. Fertilization is strictly internal.

The Skin has become dry, virtually devoid of glands. The outer layers of the epidermis undergo keratinization.

Structural Features of reptiles in relation to terrestrial life:

Dry skin, virtually devoid of glands, covered with horny scales, scutes, or plates (which serve as a means of protection against moisture loss).

Two pairs of limbs for locomotion on land.

Eyes protected by three eyelids (upper, lower, and the nictitating membrane).

A true rib cage with intercostal Muscles (providing the breathing mechanism characteristic of terrestrial animals).

Differentiation of the respiratory tract (Larynx, Trachea, two Bronchi), and a complex lung Structure (a system of septa that increase the surface area for gas exchange).

An increase in the relative size of the brain and the emergence of the rudiment of the cerebral cortex (more complex reflex activity); a highly developed Cerebellum (complex coordination of movements).

Increased complexity of the Sensory Organs.

The egg is covered by several membranes that protect the embryo from desiccation, bacterial and fungal penetration, and mechanical damage, while also providing gas exchange; they enable the direct development of reptiles on land (the larval stage is absent), as the embryo is supplied with nutrients.

Characteristics of life processes in reptiles associated with terrestrial life include exclusively pulmonary respiration and internal fertilization.

Structural features of reptiles.

The body shape of reptiles is more diverse compared to amphibians, which is associated with various modes of locomotion. Most lizards, chameleons, and crocodiles outwardly resemble caudate amphibians and exhibit the most primitive type of locomotion. In some other representatives, the limbs are located on the sides of the body, causing the body to Touch the substrate—to crawl or slither (hence the name of the class, Reptilia). Snakes and some lizard species lack limbs altogether; they are capable of moving not only through grassy thickets but also in tree crowns and swimming.

The integument of reptiles (Fig. 64): the upper layers of the Stratified Epithelium undergo keratinization, with Cells filling with keratin protein granules that displace the protoplasm and Nucleus. The proliferation of the stratum corneum gives rise to scutes and scales, which sometimes take the form of spines or claws. Beneath the horny scales in some reptile species, bony plates are located in the mesodermal layer of the skin—the corium—and in turtles, these fuse into a bony shell that is fused to the spine.

Fig. 64. Skin of reptiles (after Naumov, Kartashev, 1979):

1 — epidermis; 2 — corium; 3 — stratum corneum; 4 — Malpighian layer; 5 — pigment cells; 6 — bony plates.

Beneath the dead stratum corneum lies the lower Malpighian layer, which consists of living cells capable of division. Pigment cells are located in the Malpighian layer and the upper PARTS OF THE corium. This skin structure provides excellent protection against water loss through evaporation, mechanical damage, and the penetration of pathogens. At the same time, it has lost the capacity for gas exchange and excretion of metabolic waste. Only in crocodiles is a negligible amount of water excreted through the integument. The skin is devoid of glands. A small number of glands secreting odorous substances (which act as chemical signals for attracting the opposite sex or deterring intruders from occupied territory) are retained on the thighs and near the cloaca in lizards, and on the snout and some other body parts in crocodiles, snakes, and turtles. Shedding of the horny layer occurs through molting: either complete (in snakes) or partial (in lizards) shedding of the horny sheath and the formation of a new one. The animal grows while the new integument has not yet keratinized. In many species, molting occurs several times a year.

The skeleton of reptiles is divided into the axial skeleton, the Skull (neurocranium and viscerocranium), and the Skeleton of the limbs (forelimbs and hindlimbs) and their girdles.

The axial skeleton, formed by the vertebrae, consists of five regions:

1) cervical — 7-10 vertebrae; 2) thoracic — 12-20 vertebrae; 3) lumbar — 5 vertebrae; 4) sacral — 2 vertebrae; 5) caudal — 15-40 vertebrae.

Great mobility of the head is ensured by the differentiation of the first two cervical vertebrae—the atlas and the axis (epistropheus). The atlas has the shape of a bony ring divided by a strong ligament into upper and lower halves; the brain connects to the Spinal Cord through the upper opening; the anterior surface of the lower half articulates with the skull, while the odontoid process of the second cervical vertebra—the axis—enters the lower opening from behind. The head can turn sideways on the odontoid process, and its movement in the vertical plane is enabled by the Articulation of the skull with the atlas. All this allows for complex Head movements during foraging and orientation. Studies of embryonic development in reptiles have shown that the odontoid process of the second cervical vertebra is formed by the fusion of the atlas body to the axis.

In the only modern (but archaic) species—the tuatara (order Rhynchocephalia)—the vertebrae are of the amphicoelous type; remnants of the notochord are preserved between the vertebral bodies.

In crocodiles and most squamates, the vertebrae are of the procoelous type (the vertebral bodies are concave anteriorly and convex posteriorly), with only a few being amphicoelous. In turtles, the vertebrae are of various types: the posterior vertebrae are procoelous, the anterior ones are opisthocoelous (the vertebral bodies are convex anteriorly and concave posteriorly), and the middle ones are amphicoelous.

Reptiles develop a true rib cage formed by the thoracic vertebrae, ribs, and Sternum. The pectoral girdle attaches to the sternum. 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 skeleton of the limbs and their girdles (Fig. 65) in reptiles has almost the same structure as in amphibians.

Fig. 65. Pectoral girdle and forelimb (A) and pelvic girdle of a lizard (after Naumov, Kartashev, 1979):

A: 1 — clavicle; 2 — suprascapular Cartilage; 3 — scapula; 4 — coracoid; 5 — ribs;

6 — sternum; 7 — procoracoid cartilage; 8 — episternum; 9 — humerus; 10 — ulna;

11 — radius; 12 — carpus; 13 — metacarpus; 14 — Phalanges of digits.

B: 1 — acetabulum; 2 — ilium; 3 — pubis; 4 — cartilaginous joint.

In lizards (sand lizard, viviparous lizard) and the tuatara, the caudal vertebrae have thin cartilaginous layers in the middle, enabling them to undergo autotomy (self-amputation). If these animals are grabbed by the tail, a powerful Muscle contraction causes the caudal vertebra to fracture in the middle, and the end of the tail sheds. This reflex serves a protective function, allowing the animal to escape from a predator that has seized its tail. A new tail grows at the site of the break, meaning regeneration occurs.

In snakes and legless lizards, THE Vertebral Column is divided only into trunk and caudal regions (Fig. 66). All trunk vertebrae bear movable ribs that rest on the ventral scutes. Snakes lack a rib cage. In their skeleton, the limbs and their girdles are lost, while the number of vertebrae increases. The combination of A large number of short vertebrae acting as levers accounts for their ability to vary their curvature over a wide range, providing extraordinary body flexibility. This explains the remarkable flexibility of snakes and legless lizards.

Fig. 66. Skeleton of a snake (after Naumov, Kartashev, 1979):

1 — skull; 2 — fangs; 3 — lower jaw; 4 — trunk vertebrae with ribs; 5 — caudal vertebrae.

The musculature of reptiles is more differentiated than that of amphibians. Powerful masticatory muscles develop on the skull. The emergence of a mobile cervical region is accompanied by the development of neck musculature. The ribbon-like Muscles of the Trunk musculature participate in body bending and limb movements. Rudimentary subcutaneous musculature appears, which alters THE POSITION OF the horny scales. It is much better developed in snakes and legless lizards, where it participates in locomotion by ensuring coordinated Movements of the ventral scutes. With the appearance of a true rib cage, intercostal muscles (external and internal) emerge, providing the specialized breathing mechanism characteristic of terrestrial vertebrates.

The Digestive System of reptiles is more complex than that of amphibians. In crocodiles and turtles, the Oral Cavity is separated by a bony palate, which allows them to breathe while feeding. Food capture (reptiles

mostly feeding on invertebrates and small vertebrates) is carried out by jaws equipped with numerous sharp Teeth. The teeth are fused to the jaws (pleurodont teeth) and the bony palate (acrodont teeth); only in crocodilians are the teeth set in special jaw sockets—alveoli (thecodont teeth). In most modern reptiles, the teeth are uniform and serve only to capture and hold prey. Only some snakes develop specialized, large venomous fangs with grooves through which venom flows. The venom is produced by modified Salivary Glands. At rest, the venomous fangs fold back against the palate, but during a bite, they erect forward. Furthermore, The structure of the jaw apparatus in snakes (the mobile articulation of the BONES OF THE left and right sides of the jaws) allows them to open their mouths extremely wide and swallow prey much larger than the snake's own width (Fig. 67).

Fig. 67. Jaw apparatus of a snake: closed (A) and open (B) Mouth;

C — diagram of the position of the main levers (after Naumov, Kartashev, 1979):

1 — venomous fang; 2 — Maxilla; 3 — transverse bone;

4 — pterygoid bone; 5 — squamosal bone; 6 — quadrate bone.

Most species swallow their food whole. Crocodilians and turtles are capable of tearing individual pieces from large prey. Turtles lack teeth; their function is performed by the sharp-edged horny sheaths of the jaws.

The oral cavity of reptiles contains salivary glands that produce saliva with digestive Enzymes. At the bottom of the oral cavity is a muscular Tongue capable of extending far outward. In snakes and lizards, it is bifid (forked) at the tip and is used as a tactile organ. In chameleons, the tip of the tongue is thickened and can be instantly projected to capture small, fast-moving prey (insects).

Reptiles have a well-developed Esophagus. In snakes, it has particularly strong musculature, which helps push large prey into The Stomach. The intestine of reptiles is relatively longer than that of amphibians, especially in herbivorous species. At the boundary between the small and large intestines, There is a small cecum (which is better developed in herbivorous species). The Liver and Pancreas open into the duodenum through separate ducts. The intestine opens into the cloaca.

The functioning of the digestive system in reptiles characterizes them as thermophilic (warmth-loving) animals: the temperature optimum for their digestive enzymes is higher than that of amphibians. The Digestion of large prey by snakes proceeds normally only at a sufficiently high ambient temperature; slower digestion at low temperatures causes food poisoning and the death of the animal. A unique feature of reptiles is their ability to fast. Some turtles and snakes can live without food for 1-2 years (according to zoo observations), and lizards can survive without food for several weeks.

The excretory system of reptiles is represented by pelvic, kidney-bean-shaped Kidneys and Ureters that open into the cloaca and Urinary Bladder. The urinary bladder opens into the cloaca from the ventral side. Urine is discharged into the cloaca, then into the urinary bladder where it accumulates, and is subsequently expelled through the cloaca. A new type of excretion has evolved in reptiles—the elimination of poorly water-soluble uric acid. It is excreted in the form of small crystals ("white urine").

Sea turtles and some other reptiles that drink saltwater have special glands that excrete excess salts from the body. In turtles, these are located near the eyes. The secretion of the salt glands of marine reptiles contains a 7% solution of common salt (NaCl), which is twice the salinity of ocean water. Thus, sea turtles literally "cry bitter tears" to rid themselves of excess salt. In marine iguanas, which feed on seaweed, the salt glands exist as "nasal glands" that open via ducts into the Nasal cavity. Salts are excreted as droplets of secretion from the nostrils.

The Respiratory system of reptiles consists of the airways (nasal openings—nostrils, larynx, trachea, two bronchi) and lungs. The larynx is supported by cartilages. The trachea has walls made of elastic cartilaginous rings.

The walls of the bronchi are also reinforced with cartilaginous rings. The lungs have a system of septa (partitions) that significantly increase the surface area for gas exchange (Fig. 68). In chameleons, some lizards, and snakes, the posterior part of the lungs has thin-walled, finger-like projections: no oxygenation of blood occurs in their walls; instead, they act as air reservoirs, produce a hissing effect, and facilitate gas exchange during the prolonged passage of food through the esophagus and during diving.

Fig. 68. Lungs of reptiles (after Naumov, Kartashev, 1979):

A — amphisbaena; B — anaconda; C — tuatara; D — monitor lizard; E — alligator; F — chameleon.

The breathing mechanism of reptiles: air enters and leaves the lungs due to the action of intercostal and Abdominal muscles, the contraction of which changes the volume of the internal body cavity.

The respiratory rhythm of reptiles changes depending on the ambient temperature and the animal's state. In a lizard, the breathing rate at 15°C is 26 breaths per minute, at 25°C it is 31, and at 35°C it is 37.

The skin of reptiles, covered with horny structures, does not participate in respiration.

In aquatic turtles, accessory respiratory organs in water include vascularized outgrowths of the Pharynx and cloaca (anal sacs), which are rich in capillaries.

The Circulatory system OF reptiles is similar in structure to that of amphibians. The Heart is three-chambered. The atria are separated by a complete septum; each opens into the ventricle through an independent opening,

which is guarded by a valve of semilunar cusps. The ventricle has an incomplete septum that divides it into two parts: at the moment of contraction (systole), the septum temporarily divides it completely, which is crucial for separating blood with different oxygen content. In crocodilians, the septum is complete (a four-chambered heart), but there is an opening in the middle.

Reptiles have two circuits of Blood Circulation. The pulmonary (minor) circuit: the pulmonary artery arises from the right side of the ventricle and divides into the right and left Arteries, which carry deoxygenated (venous) blood to the lungs. Gas exchange occurs in the pulmonary capillaries, converting deoxygenated blood into oxygenated (arterial) blood. The left atrium receives the pulmonary vein, formed by the fusion of the right and left Pulmonary Veins (carrying oxygenated blood). The systemic (major) circuit: the right aortic arch arises from the left side of the ventricle, branching into the carotid and subclavian arteries, which carry oxygenated blood to the brain; the left aortic arch arises from the middle part of the ventricle. Behind the heart, the left and right aortic arches merge into the dorsal aorta, which contains mixed blood with a predominance of oxygenated blood. The dorsal aorta branches into arteries that carry mixed blood to all Organs of the body. Veins from the body organs form the right and left anterior venae cavae, which empty into the right atrium.

Reptiles are poikilothermic animals, but unlike amphibians, their body temperature during active states is relatively constant and fluctuates within narrower limits than the ambient temperature. Reptiles use solar radiation to raise their body temperature. The temperature optimum for reptile activity is between 20 and 38°C. During cooling, blood sugar in reptiles helps maintain body temperature for some time by increasing heat production (chemical thermoregulation). There is one endothermic (warm-blooded) reptile species on Earth—the leatherback sea turtle.

The nervous system of reptiles is more complex compared to that of amphibians. The brain of reptiles, like that of amphibians, consists of five divisions, but the cerebral hemispheres of the Forebrain are significantly larger, and a cortex (grey matter) begins to form on their surface. The cerebellum is well-developed.

The sensory organs of reptiles include:

eyes protected by three eyelids; in snakes and some lizards (geckos, skinks), the eyelids fuse to form a transparent membrane; in nocturnal species, the eyes are enlarged and have vertical pupils; lacrimal glands protect the eyes from drying out in the air; accommodation of the eye is achieved by moving the lens and changing its curvature using the ciliary muscle; the retina of nocturnal animals contains only rods, while the retina of diurnal species has both rods and cones (Color Vision); the sensitivity of color vision is shifted toward the orange part of the spectrum; unlike amphibians, the analysis and integration of visual sensations occur not in the retina, but primarily in the visual cortex of the Midbrain;

thermal Sense Organs — thermoreceptors and even thermal detectors: in rattlesnakes, on the sides of the snout, between the nostrils and the eyes, there are paired pits capable of detecting temperature changes of 0.02°C from a distance of up to

15 cm; in the dark, these organs help rattlesnakes find warm-blooded prey;

Hearing organs, similar to those of amphibians, but their tympanic membrane is located in a small depression; reptiles perceive sounds in the range of 20-6000 Hz, but hear well only in the range of 60-200 Hz; in snakes, hearing is poorly developed: they lack a tympanic membrane and primarily perceive vibrations propagating through the substrate (ground) or water (seismic hearing); in turtles, hearing is also poorly developed: their tympanic membrane is thick, and in some species, the auditory canal is covered by thickened skin;

organs of touch — sensitive receptors on keratinized skin cells and the tip of the tongue;

olfactory organs open externally through paired nostrils, and into the oral cavity through slit-like choanae; in front of the choanae is Jacobson's organ, which detects the smell of food in the oral cavity.

Reptiles are dioecious animals. Females have paired Ovaries, and males have Testes. The ovaries look like granular oval bodies. The oviducts are represented by the Müllerian ducts. They begin with ciliated funnels located near the ovaries, followed by the albuminous region and the Uterus, which opens into the cloaca. The testes also have the appearance of oval bodies. Through the epididymides, which represent the remaining part of the mesonephros (trunk kidney) and contain numerous tubules, the testes are connected to the vas deferens (Wolffian ducts). The right and left vasa deferentia open into the corresponding ureters, which open into the cloaca. Fertilization is internal. Consequently, males of all reptiles, except the tuatara, have specialized copulatory organs: in crocodiles and turtles, this is a single organ, while in lizards and snakes, these are paired outgrowths of the posterior wall of the cloaca that evert during mating. Sexual maturity in reptiles is reached at different times: in crocodiles and most turtles, at 6-10 years of age; in snakes, at 3-5 years; in large lizards, at 2-3 years; and in small lizards, at 9-10 months of age.

Fertilization occurs in the upper part of the oviduct. Secretions from the Glands of the middle part of the oviduct (albuminous region) form an albumen coat around the egg Cell (yolk), which is poorly developed in snakes and lizards, and well-developed in turtles and crocodiles. The external membranes — fibrous and leathery shell membranes — are formed from the secretion produced by the Cells of the lower part of the oviduct (uterus).

In a small number of species (slow worm, common adder, viviparous lizard, sea snakes, some colubrid snakes and lizards), ovoviviparity is observed: fertilized eggs are retained in the female's reproductive tract, undergoing all Selection/3.html">Stages of development there; the embryos hatch immediately after the female lays the eggs. True viviparity is known in some skinks. They lack an outer eggshell, and the embryonic membranes of the fetus lie adjacent to the walls of the uterine section of the oviduct; through osmosis and diffusion, oxygen and nutrients from the mother's bloodstream enter the embryo's circulatory system. In some colubrids and lizards, a true Placenta is formed: outgrowths of the embryo's serosa and allantois embed into the mucous membrane of the uterine section of the oviduct. Due to the close apposition of the maternal and embryonic Blood Vessels, The transport of oxygen and nutrients to the embryo is facilitated.

In some lizards (Caucasian rock lizards, North American teiids, agamas, geckos), parthenogenetic reproduction has been established, which is the development of unfertilized eggs. Populations of these species consist solely of females. The existence of an all-female, unisexual population is advantageous: it allows limited food resources to be used most efficiently, solely by individuals that ensure the continuation of the species.

In the golden lancehead snake, which lives exclusively on Queimada Grande Island with an area of only 3 km² (60 km from the city of Santos in Southern Brazil), an amazing case of Hermaphroditism has been documented. In a small island population, such hermaphroditism likely helps increase reproduction rates without increasing the number of individuals.

The fecundity of reptiles is lower than that of amphibians. Parental care is observed among reptiles. Most reptiles bury their eggs in soil that is well-warmed by sunlight; some species lay eggs in piles of plant debris or under decaying stumps, utilizing the heat generated during decomposition. Some crocodiles dig pits and cover them with plant debris; females stay near the nest and guard the clutch. When the young hatch, the female crocodile digs up the clutch, facilitating their emergence to the surface. Monitor lizards also guard their clutches. Female pythons coil their bodies around the clutch of eggs, not only protecting it but also incubating it: in such a nest, the temperature is 6-12°C higher than the ambient temperature.

The annual Life Cycle of reptiles is similar to that of amphibians and, under seasonal climate conditions, is divided into the following periods: spring emergence, breeding season, period of summer activity, and overwintering (during which physiological processes slow down drastically). In steppes and deserts, extremely high summer temperatures prompt reptiles to enter so-called estivation (summer dormancy).