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

CHAPTER 4. SUPERCLASS TETRAPODA. LOWER TERRESTRIAL VERTEBRATES

4.1. Class Amphibia - Amphibians

Amphibians are the first primitive terrestrial vertebrates belonging to the anamniote group. Their individual development is characterized by a habitat shift: the early developmental stages take place in Water, whereas adult animals are adapted to life on land. Consequently, amphibians lead a semi-aquatic (water-land) lifestyle. They exhibit both primitive features that affinity them with fish and complex adaptations

characteristic exclusively of terrestrial animals.

Similarities between amphibians and aquatic animals:

1. similar egg development;

2. absence of embryonic membranes;

3. presence of a larval stage;

4. respiration in larvae and many aquatic amphibians occurs via gills;

5. similar Structure OF THE circulatory systems in amphibian larvae and fish;

6. presence of the conus arteriosus and aortic arches;

7. similar excretory system structure: pronephric in larvae, mesonephric in adults (imago);

8. poikilothermy;

9. mucous and water-permeable Skin;

10. identical number of Cranial Nerves (10 pairs).

Features of amphibian Organization:

1. loss of the exoskeleton;

2. platybasic, autostylic Skull (the palatoquadrate element fuses with the neurocranium and articulates with the spine via two occipital condyles), significantly lightened;

3. tubular structure of lever-type limbs;

4. Bone Marrow producing a significant amount of Hemoglobin;

5. system of flexor and extensor Muscles;

6. Middle ear containing a single ossicle;

7. cutaneous-pulmonary respiration;

8. movable eyelids;

9. a mixed Circulatory system with two undivided circulatory loops;

10. a three-chambered Heart consisting of two atria and one ventricle, with an conus arteriosus;

11. two cerebral hemispheres;

12. the cerebral vault - the archipallium.

General characteristics of amphibians using the marsh frog as an example.

The Class Amphibia is the least numerous among gnathostome vertebrates. It numbers just over 3,400 species, which are grouped into three orders, and those, in turn, into two subclasses:

Body shape. Modern amphibians have a short body, flattened dorso-ventrally with a reduced tail, hind limbs longer and stronger than the forelimbs (order Anura); or a cylindrical, elongated body, sometimes slightly flattened or laterally compressed, with a small HEAD, a long tail, and short limbs (order Urodela); finally, some Representatives of the class are limbless, worm-like animals with a small head (order Apoda) (Fig. 145). The sizes of amphibians vary: anurans range from 3–25 cm in length; urodeles from 10–30 cm, with some large species (the giant salamander reaching up to 1.6 m); caecilians (Apoda) reach lengths of 30–120 cm.

Fig. 145. General structure of a frog (A):

1 - choanae; 2 - eyelids; 3 - eye; 4 - tympanic membrane; structure of the frog's forelimbs (B): a - upper arm; b - forearm; c - hand and digits; structure of the hind limbs (C): a - thigh; b - lower leg; c - FOOT and digits; structure of a newt (G) and a caecilian (D): 1 - head; 2 - body;

3 - tail; 4 — limbs

On the head, in front of the eyes, clearly visible nostrils are present, which are closed by Valves and connected to the olfactory capsules; the latter open into the Oral Cavity through openings called choanae, or internal nostrils. Large eyes are located on the sides of the head, equipped with upper and lower eyelids typical of terrestrial animals. In addition, the eye is covered by a third eyelid, situated beneath the lower one and known as the nictitating membrane. Behind the eyes lie rounded openings covered by thin tympanic membranes. Unlike females, male frogs have vocal sacs (resonators) at the corners of the Mouth that open into the oral cavity. The short, four-toed forelimbs of frogs are noticeably smaller than the large, five-toed hind limbs, between the digits of which webbed membranes are stretched. All digits lack claws. At the Base of the first (innermost) digit on the forelimbs of males, There is a large tubercle that develops particularly during the breeding season, helping to hold onto the female during Fertilization of the eggs. At the posterior end of the body lies a single excretory opening - the cloaca (Fig. 146).

Fig. 146. Organs located on the frog's head:

1 - mouth; 2 - external naris; 3 - upper eyelid; 4 - lower eyelid; 5 - tympanic membrane; 6 - external vocal sacs; 7 - Tongue; 8 - choanae; 9 - openings of the Eustachian tubes; 10 - laryngeal cleft; 11 - vomerine Teeth; 12 — eye

Integument, skin, and their derivatives. The epidermis is multilayered, and the corium is thin but richly supplied with capillaries. The skin of amphibians features numerous multicellular glands (Fig. 147).

Fig. 147. Structure of frog skin:

1 - epidermis; 2 - corium; 3 - glandular Cells of the skin gland; 4 - muscular layer of the gland; 5 - excretory duct of the skin gland; 6 - pigment cells

They secrete a thin layer of mucus that covers the entire body, moisturizing the skin and thereby protecting it from desiccation. The skin participates in gas exchange. In toads living in relatively dry habitats, the mucus thickens and forms a dense film on the skin, which reduces water loss. The secretion of the Skin glands (in fire-bellied toads, true toads, and some salamanders) contains irritants or toxic substances. The secretion also contains substances with signaling Functions, through which they influence The behavior of other individuals. Pigment cells located in the lower layers of the epidermis and in the corium determine the species-specific coloration. The latter performs various functions: concealment (cryptic or protective coloration); warning and deterrence in species with poisonous glands (aposematic coloration with bright color patches); and Sexual Dimorphism - in males, the coloration quite often becomes brighter at the onset of breeding, which facilitates the encounter of sexually mature individuals and stimulates mating.

Individual species can change the intensity of their coloration depending on the Background color, which is best expressed in certain tree frogs. In terrestrial species, the flat cells of the outer epithelial layer keratinize at the tips of the digits to form claws (African clawed frog, clawed newt). In caecilians, the corium contains small bony scales - remnants of the bony armor of Paleozoic amphibians. In anurans, large lymphatic lacunae (reservoirs) are located beneath the skin, where a water reserve accumulates under favorable conditions.

The Skeleton of amphibians consists of THE Vertebral Column or axial trunk, the skull, the limbs, and their girdles (including the Pelvic Girdle). The vertebral column is divided into 4 regions: cervical, trunk, lumbar (or sacral), and caudal (Fig. 148).

Fig. 148. Skeleton of a salamander (A) and a frog (B):

1 - cervical vertebra; 2 - trunk vertebrae; 3 - sacral vertebra; 4 - trunk vertebrae (fused into a single bone in the frog - the urostyle);

5 – Pectoral Girdle; 6 – upper arm; 7 – radius; 8 – ulna; 9 – fused ulna and radius; 10 – carpus;

11 – metacarpus; 12 – Phalanges of the digits; 13 – thigh; 14 – Tibia; 15 – Fibula; 16 – fused tibia and fibula; 17 – tarsus; 18 – metatarsus; 19 – phalanges of the digits

The cervical and lumbar regions each have a single vertebra. The first vertebra provides a certain degree of head mobility relative to the trunk, while the lumbar vertebra serves for articulation with the pelvic girdle. Anuran amphibians have 7 trunk vertebrae; all caudal vertebrae (12 in total) fuse into a single bone called the urostyle. Urodeles possess 13–62 trunk and 22–36 caudal vertebrae, whereas the total number of vertebrae in caecilians reaches 200–300.

In primitive amphibians, the vertebrae are amphicoelous, much like those in fish, with remnants of the notochord persisting between and within the vertebral bodies. In true salamanders and some anurans, the vertebrae are opisthocoelous (convex anteriorly and concave posteriorly), whereas in the remaining anurans, they are procoelous (concave anteriorly and convex posteriorly). Well-developed neural arches are situated above the vertebral bodies, forming the canal for the Spinal Cord. The trunk vertebrae feature well-developed transverse processes, to which very short Ribs attach in tailed amphibians. The transverse processes of the cervical vertebra are poorly developed.

The adult amphibian skull contains a large amount of Cartilage. Compared to teleost fish, modern amphibians have fewer bones, whereas ancient extinct species possessed more dermal bones. In the occipital region of the axial skull, paired lateral occipital bones develop (Fig. 149), bearing occipital condyles.

Fig. 149. SKULL OF A frog:

A – dorsal view (dermal BONES OF THE braincase removed on the right); B – ventral view;

1 – premaxilla; 2 – Maxilla; 3 – frontal process of the maxilla; 4 – quadratojugal bone;

5 – squamosal bone; 6 – prootic bone; 7 – occipital joint; 8 – frontoparietal bone; 9 – Nasal bone; 10 – sphenethmoid bone; 11 – preorbital process; 12 – pterygoid bone; 13 – frontal fontanelle; 14 – parietal fontanelle; 15 – exoccipital bone; 16 – Vomer; 17 – parasphenoid; 18 – Palatine bone; 19 – palatoquadrate cartilage

Two occipital condyles articulate with the cervical vertebra, which is a characteristic feature of amphibians. Reptiles, birds, and mammals possess a single condyle. The auditory region contains a single pair of bones, the prootic. In the orbital region of tailed amphibians, there are paired orbitosphenoid bones; in anurans, these fuse into a single ring-shaped sphenethmoid bone. The remaining part of the braincase remains cartilaginous. There is also a relatively small number of dermal bones. The skull roof is formed by paired parietal bones, which fuse into paired frontoparietal bones in anurans. Paired nasal bones (nasale) lie anteriorly, accompanied by an additional pair of prefrontal bones in urodeles. A dermal squamosal bone forms in the auditory region. The floor of the skull is covered by a large parasphenoid, anterior to which lie the paired dermal palatine and vomer bones; in tailed amphibians, these fuse into paired palatovomer bones. Small teeth are present on the vomers, and in urodeles, on the palatine bones as well.

In the visceral region of the skull, the palatoquadrate cartilage persists throughout life. Its anterior and posterior ends are fused to the braincase (autostyly) (Fig. 149). Paired dermal bones—the premaxilla and the maxilla—adhere to the palatoquadrate cartilage. Small teeth are present on these jaw bones; in some species, such as frogs, they are reduced. The posterior region of the palatoquadrate cartilage is covered dorsally by the dermal quadratojugal and squamosal bones, and ventrally by the pterygoid bone.

In some tailed amphibians, the posterior region of the palatoquadrate cartilage ossifies to form a small quadrate bone. The primary lower jaw, Meckel's cartilage, remains cartilaginous, with only its anterior end ossifying into small paired mentomeckelian bones. Posterior to them, overlying Meckel's cartilage, lie dermal dentary bones, which are toothless in modern amphibians. The posterior part of Meckel's cartilage is surrounded by an elongated dermal angulosplenial bone and several small accessory dermal ossicles. Meckel's cartilage articulates with the posterior end of the palatoquadrate cartilage via its articular process, forming the jaw joint. The complete reduction of the operculum led to the loss of the hyoid arch's primary functions.

The paired limbs of amphibians differ significantly in Morphology and function from the paired fins of fish. They possess only an internal skeleton and are structured as lever systems connected by hinge joints. The complex structure of the girdles provides the limbs with a firm support, reinforcing their attachment to the trunk. Both pairs of limbs follow a unified structural plan shared by all tetrapods (Fig. 150).

Fig. 150. Hind limb (A) and pelvic girdle (B) of a frog in lateral view:

1 – Femur; 2 – femoral head; 3 – crus (shank); 4 – tibia; 5 – fibula; 6 – tarsus;

7 – tibiale; 8 – fibulare; 9 – metatarsus; 10 – phalanges of the digits; 11 – rudimentary digit; 12 – ilium; 13 – ischium; 14 – pubic cartilage; 15 – acetabulum; I–V – digits.

Forelimb and pectoral girdle of a frog (C): 1 – humerus; 2 – humeral head; 3 – forearm; 4 – ulna; 5 – radius; 6 – olecranon process; 7 – carpus; 8 – metacarpus; 9 – phalanges of the digits; 10 – scapula; 11 – suprascapular cartilage; 12 – coracoid;

13 – glenoid fossa for the humeral head; 14 – procoracoid cartilage; 15 – clavicle; 16 – Sternum; 17 – episternum; I – reduced first digit; II–V – well-developed digits

The pectoral girdle in amphibians resembles a semi-ring embedded within the trunk musculature. In anurans, the ossified scapula and coracoid form the glenoid cavity for articulation with the head of the humerus (Fig. 150). A broad suprascapular cartilage attaches to the scapula, serving as an attachment site for back muscles. Anterior to the coracoid lies a slender, rod-like procoracoid cartilage. Ventrally, it is covered by a thin dermal bone, the clavicle. Posterior to the junction of the coracoids lies the bony sternum, which terminates in a cartilaginous expansion. Anterior to the clavicles lies a small episternum; its anterior part is cartilaginous, while the posterior part ossifies. Amphibians lack a rib cage. Urodele amphibians possess short ribs that do not reach the sternum.

The pelvic girdle consists of three elements that join together to form the acetabulum—the socket for the Articulation of the femoral head; in anurans, the girdle is almost completely ossified. The long iliac bones attach to the transverse processes of the sacral vertebra; the ischial bones fuse with each other, and the pubic cartilage lies ventrally. In tailed amphibians, the ischium and ilium are relatively small in size.

The Muscular System of amphibians differs substantially from that of fish. Part of the trunk musculature retains a segmental (metameric) structure. However, clear differentiation is observed: distinct sections of Muscle segments fuse into ribbon-like muscles. The mass of the limb muscles increases dramatically, represented by a complex system of antagonists: flexors and extensors of the joints. The musculature of the oral cavity also becomes more complex and specialized.

Digestive System and feeding. All modern adult amphibians are carnivorous. They feed on various small invertebrates: insects and their larvae, centipedes, spiders, crustaceans, worms, etc. Aquatic species may catch fish fry, while the largest among them prey on other amphibians and their larvae, waterfowl chicks, and small rodents that have fallen into the water.

Caecilians lead a burrowing lifestyle, feeding on soil invertebrates, with some species settling in termite mounds and anthills to consume their inhabitants. Their foraging Methods are relatively uniform. Caecilians move slowly, locating prey through SENSE OF SMELL and Touch, whereas tailed amphibians rely on sight and smell. Anuran amphibians rely primarily on Vision and occasionally on sense of smell when hunting. They locate prey by moving in slow hops or, more frequently, by sitting and waiting. They capture prey by projecting a sticky tongue.

Some species capture prey with their jaws; others, such as the African clawed frog and green toads, occasionally push prey into their mouths using the digits of their forelimbs. Strong-leaping frogs (species of the genus Rana and others) are also capable of catching flying toads and insects. Sluggish toads feed primarily on crawling insects (beetles, ants, caterpillars, etc.).

In amphibians, the oral opening is wide and leads into the oro-pharyngeal cavity, which transitions into the Esophagus (Fig. 151).

Fig. 151. Internal anatomy of a frog (female)

1 - heart; 2 - Lungs; 3 - left lobe of the Liver; 4 - right lobe of the liver; 5 - Gallbladder in the central lobe of the liver; 6 - Stomach;

7 - Pancreas; 8 - duodenum; 9 - Small Intestine; 10 - rectum; 11 - Spleen; 12 - cloaca (dissected); 13 - Urinary Bladder; 14 - opening of the urinary bladder; 15 - Kidney; 16 - Ureter; 17 - paired openings of the Ureters in the cloaca; 18 - right Ovary (left removed); 19 - fat body; 20 - right oviduct; 21 - left oviduct; 22 - uterine section of the oviduct; 23 - opening of the oviduct in the cloaca; 24 - dorsal aorta; 25 - posterior vena cava; 26 - common carotid artery; 27 - left aortic arch; 28 - pulmocutaneous vein

In the anterior part of the palate within the oro-pharyngeal cavity, the paired choanae—internal nostrils—open, while near the jaw joint are the openings of the Eustachian tubes, which lead to the middle ear cavity. In the males of some frog species, vocal sacs (resonators) are located at the corners of the mouth to amplify sound. The floor of the posterior part of the oro-pharyngeal cavity is supported by the cartilages of the laryngeal cleft. Mucous glands are located in the mucous membrane of the roof of the oro-pharyngeal cavity; they secrete a mucous substance devoid of digestive Enzymes that moistens the cavity and facilitates the swallowing of prey. Saliva-moistened food is moved into the esophagus by the contraction of muscles in the floor of the oro-pharyngeal cavity, assisted by the eyes, which retract into the oro-pharyngeal cavity. The esophagus is short, highly distensible, and empties into The Stomach; it possesses thick muscular walls. Extending from the stomach is the duodenum, which gradually transitions into the small intestine, emptying into the rectum. The latter opens into the cloaca. The trilobed liver features a gallbladder situated within its central lobe, with the Bile duct emptying into the duodenum. The pancreas lies between the stomach and the duodenum. Near the posterior end of the stomach is the spleen, an organ of hematopoiesis and a Blood reservoir.

Respiratory organs and gas exchange. A characteristic feature of amphibians is the presence of multiple respiratory organs. Oxygen uptake and carbon dioxide release involve: in larvae—the skin, external, and internal gills; in adults—the lungs (Fig. 152), skin, and mucous membrane of the oro-pharyngeal cavity. In some species of adult tailed amphibians, external gills and underdeveloped or reduced lungs are retained. Adult amphibians breathe using paired lungs, which appear as hollow sacs with honeycombed internal walls (absent in some tailed amphibians) (Fig. 152).

Fig. 152. Respiratory system of a frog (A):

1 - lungs, 2 - esophagus, 3 - Pharynx, 4 - external nostrils, 5 - lower jaw, 6 - tongue, 7 - glottis,

8 - vocal cords, 9 - Bronchi, 10 - stomach.

Diagram of amphibian lung structure (B): 1 - newt; 2 - frog; 3 — toad

Lung ventilation is accomplished through Movements of the floor of the oro-pharyngeal cavity (Fig. 153).

Fig. 153. Mechanism of frog lung ventilation

Upon lowering the floor, air is drawn through the nostrils into the oro-pharyngeal cavity; subsequently, the external nostril openings close, the glottis opens, and air is forced out of the lungs into the oro-pharyngeal cavity under pressure from the body wall muscles and Internal Organs, where it mixes with atmospheric air. The floor of the cavity gradually rises, pressing against the palate, and the mixed air is forced into the lungs. The glottis then closes, and the remaining air is expelled outward through the nostrils.

Circulatory system and Blood Circulation. Amphibian larvae possess a single circulatory loop: The Heart contains one atrium and one ventricle, along with an truncus arteriosus that branches into four afferent branchial Arteries. The adult amphibian heart is three-chambered, consisting of two atria and one ventricle (Fig. 154).

Fig. 154. Diagram of the frog heart:

1 - right atrium, 2 - left atrium, 3 - ventricle, 4 - valves closing the common opening leading from both atria into the ventricle, 5 - truncus arteriosus, 6 - common arterial trunk, 7 - pulmocutaneous artery, 8 - aortic arch, 9 - common carotid artery, 10 - carotid gland

A thick-walled sinus venosus adjoins the right atrium, and the truncus arteriosus extends from the ventricle. Thus, the heart comprises five sections. Both atria open into the ventricle via a common opening equipped with atrioventricular valves, which prevent blood from flowing back into the atria during ventricular contraction. The ventricle features muscular wall outgrowths that form a series of interconnected chambers, preventing the mixing of blood. The truncus arteriosus departs from the right side of the ventricle; a long spiral valve is located within it. Three pairs of arterial arches originate from the truncus arteriosus via independent openings.

The first vessels to branch off from the truncus arteriosus are the right and left pulmocutaneous arteries—homologues of the IV pair of larval branchial arches; these divide into the pulmonary and cutaneous arteries. The second are the systemic aortic arches (roots)—homologues of the II pair of larval branchial arches. After branching off the occipitovertebral and subclavian arteries, which supply blood to the trunk musculature and forelimbs, they unite beneath the vertebral column to form the dorsal aorta. The dorsal aorta gives rise to the major celiacomesenteric artery (supplying the digestive tract); via other Branches of the dorsal aorta, blood is distributed to the remaining organs and hindlimbs. The final vessels originating from the truncus arteriosus are the common carotid arteries—homologues of the I pair of larval branchial arches. Each of these divides into external and internal carotid arteries.

Venous blood from the posterior region of the body and hindlimbs is collected by the femoral and sciatic Veins, which unite into paired iliac or renal portal veins that break down into capillaries within the Kidneys, thereby forming the renal portal system. Veins branching off from the right and left femoral veins merge to form the unpaired abdominal vein, which runs along the abdominal wall to the liver, where it breaks down into capillaries. Venous blood from all sections of the intestine and stomach is collected into the large HEPATIC PORTAL VEIN, which branches into capillaries within the liver (in all amphibians, the hepatic portal system is formed by the abdominal and portal veins). The renal capillaries converge into numerous efferent veins that empty into the unpaired posterior vena cava; veins from the reproductive organs also empty into it. The posterior vena cava passes through The Liver and drains into the sinus venosus (Fig. 155).

Fig. 155. Venous system of a frog (A):

1 - sinus venosus, 2 - right atrium, 3 - left atrium, 4 - ventricle, 5 - External Jugular Vein, 6 - Internal jugular vein, 7 - great cutaneous vein, 8 - brachial vein, 9 - subclavian vein, 10 - right anterior vena cava, 11 - left anterior vena cava, 12 - femoral vein, 13 - sciatic vein, 14 - common iliac vein, or renal portal vein, 15

- abdominal vein, 16 - hepatic portal vein, 17 - efferent renal veins, 18 - posterior vena cava, 19 - hepatic vein, 20 - pulmonary vein, 21 - lung, 22 - liver, 23 - stomach, 24 - intestine, 25 - Testis, 26 - kidney.

Arterial System of a frog (B):

1 - right atrium, 2 - left atrium, 3 - ventricle, 4 - conus arteriosus, 5 - truncus arteriosus, 6 - pulmocutaneous artery, 7 - pulmonary artery, 8 - great cutaneous artery, 9 - right aortic arch, 10 - left aortic arch, 11 - occipitovertebral artery, 12 - Subclavian Artery, 13 - dorsal aorta, 14 - celiacomesenteric artery, 15 - urogenital arteries, 16 - common iliac artery, 17 - common carotid artery, 18 - Internal Carotid Artery, 19 - External Carotid Artery, 20 - carotid labyrinth ("gland"), 21 - lung, 22 - liver, 23 - stomach, 24 - intestine, 25 - testis, 26 - kidney

Arterial blood oxygenated in the skin collects into the great cutaneous vein, which, together with the brachial vein carrying venous blood from the forelimb, flows into the subclavian vein. The subclavian veins merge with the external and internal jugular veins into the right and left anterior venae cavae, emptying into the sinus venosus. Venous blood from all PARTS OF THE body flows through the venae cavae, along with arterial blood passing through the cutaneous veins. From the sinus venosus, blood enters the right atrium. Arterial blood from the lungs collects into the Pulmonary veins and empties into the left atrium. During pulmonary respiration, mixed blood gathers in the right atrium, while the left atrium is filled with arterial blood from the lungs. Upon simultaneous contraction of the atria, blood enters the ventricle, where its mixing is prevented by folds (ridges) of the ventricular walls: the blood is more venous in the right side of the ventricle and arterial in the left. The conus arteriosus originates from the right side of the ventricle; therefore, when the ventricle contracts, more venous blood initially enters the conus arteriosus, filling the pulmocutaneous arteries. With prolonged ventricular contraction, pressure in the conus arteriosus rises, the spiral valve shifts, and the openings of the aortic arches open, admitting mixed blood from the central part of the ventricle. When the ventricle contracts fully, the most arterial blood from the left half of the ventricle enters the conus. It cannot pass into the pulmocutaneous arteries and arches as they are already filled with blood. The pressure of the blood, maximally compressing the spiral valve, opens the orifices of the carotid arteries, through which arterial blood flows toward the head.

Excretory organs and water-salt balance. The transition of amphibians to land had a significant impact on The Nature of their water-salt balance and The excretion of nitrogenous metabolic wastes. Amphibian larvae possess pronephric kidneys (pronephroi). During metamorphosis, paired mesonephric (trunk) kidneys develop. The kidneys appear as compact bodies located on either side of the vertebral column in the lumbar region (Fig. 156).

Fig. 156. Urogenital System of a male frog (A):

1 - kidney, 2 - ureter (serving also as the vas deferens), 3 - cloacal cavity, 4 - urogenital aperture, 5 - urinary bladder, 6 - opening of the urinary bladder,

7 - testis, 8 - efferent ductules, 9 - Seminal Vesicle, 10 - fat body, 11 - Adrenal gland. Urogenital system of a female frog (B):

1 - kidney, 2 - ureter, 3 - cloacal cavity, 4 - urinary aperture, 5 - urinary bladder, 6 - opening of the urinary bladder, 7 - left ovary (the right ovary is not shown in the figure), 8 - oviduct, 9 - ostium of the oviduct, 10 - fat body (the fat body on the right side is not shown),

11 - adrenal gland, 12 - genital aperture (opening of the oviduct).

A ureter (Wolffian duct) extends from each kidney and empties into the cloaca. At the floor of the cloaca lies an opening leading to a highly distensible urinary bladder. Adrenal Glands, which are Endocrine glands, are situated on the ventral surface of the kidneys. The kidneys receive arterial blood from the dorsal aorta via renal arteries, while a significant amount of venous products of protein breakdown in adult amphibians are excreted primarily as urea (and as ammonia in larvae). As the urinary bladder fills with urea, the muscles of its walls contract, and the concentrated urine is discharged into the cloaca and expelled outward. Some metabolic wastes are excreted through the skin. The skin plays a major role in water balance. At high humidity (when moving through dew-covered grass), amphibian skin absorbs water, which accumulates in the subcutaneous glands.

Reproductive System and breeding characteristics. Amphibians are dioecious and possess paired Gonads. In most amphibians, reproduction takes place in water, although some species exhibit specialized processes (Fig. 157).

Fig. 157. Examples of terrestrial (non-aquatic) development in amphibians:

1 - female Ceylon caecilian (Ichthyophis) coiled around a clutch of eggs laid in a burrow; 2 - nest of the South American tree frog (Phyllomedusa) above water; 3 - nest of the Javan flying frog (Rhacophorus); 4 - marsupial tree frog (Gastrotheca marsupiata) with eggs in a dorsal brood pouch (pouch open); 5 - Surinam toad (Pipa pipa) with a network of brood cells on its back; 6 - male Darwin’s frog (Rhinoderma darwinii) with metamorphosing tadpoles in its vocal sac.

The Ovaries have a granular structure, are suspended by mesenteries, and by spring fill almost the entire body cavity. Multi-lobed fat bodies lie adjacent to the ovaries, accumulating nutrient reserves for The formation of Gametes during hibernation. The slender, elongated oviducts represent the Müllerian ducts. Each oviduct opens into the body cavity via a funnel (ostium) located in the cardiac region; the lower part of the funnel is expanded and opens into the cloaca. The Wolffian ducts function as ureters. Mature eggs, released through ruptured ovarian walls into the body cavity, are captured by the edges of the funnels, pass down the oviducts where they become coated with gelatinous protein layers, and accumulate in the uterine regions. The Testes are rounded and, together with the fat bodies, hang from mesenteries near the anterior margins of the kidneys. Several fine efferent ductules extend from each testis, entering the kidney and opening into the Wolffian duct. In male amphibians, the Wolffian duct simultaneously serves as both a ureter and a sperm duct (vas deferens). In the lower part of the Wolffian duct, a Swelling forms—the seminal vesicle—which enlarges sharply prior to breeding and acts as a sperm reservoir. The Wolffian ducts open into the cloaca via apertures. In male tailed amphibians (urodeles), rudimentary Müllerian ducts persist as slender tubes.

In tailless amphibians (anurans), fertilization is external. Females spawn eggs, and males immediately shed sperm over them. After some time, a larva emerges, which undergoes a series of metamorphoses (Fig. 158).

Fig. 158. Development of the Moor frog:

1 - eggs; 2 - tadpole at the moment of hatching; 3 - development of fin folds and external gills; 4 - stage of maximum development of external gills; 4a - anterior part of the same tadpole from below (larval adhesive organs are visible); 4b - structural details of external gills; 5 - stage of external gill disappearance and reduction of adhesive organs, development of the tadpole mouthparts (5a); 6 - stage of hindlimb emergence; 6a - tadpole mouthparts at the same stage; 7 - stage of hindlimb segmentation and mobility (forelimbs are visible through the skin of the gill chamber); 8 - stage of rupture of the gill chamber, release of forelimbs, metamorphosis of the mouthparts, and onset of tail resorption; 9 - emergence onto land.

In some tailed amphibians, fertilization is also external, yet peculiar. For example, in the salamander family Hynobiidae, the female deposits eggs into a gelatinous sac upon which the male deposits a spermatophore—a packet of spermatozoa bordered by a gelatinous envelope. For most tailed amphibians, fertilization is internal. The male newt deposits a spermatophore, and the female captures it with the edges of her cloaca. In caecilians (gymnophIONs), fertilization is internal: the male presses his everted cloaca against the external cloacal opening of the female, introducing seminal fluid into it. Most amphibians lay their eggs in water, where the embryo develops. The hatched larva leads an aquatic lifestyle until metamorphosis. In anurans, the formed larva escapes from the membranes and attaches to aquatic plants using a sucker. In urodeles, the larva hatches in a more advanced state of development, possessing a better-developed tail and external gills (Fig. 158).

Central Nervous System and Sensory Organs. The transition to a terrestrial lifestyle is accompanied by the restructuring of the central nervous system and sensory organs. The relative size of the amphibian Brain does not increase markedly compared to that of Fishes. Anurans have a slightly larger brain than urodeles. In modern amphibians, there is a noticeable increase in the relative size of the Telencephalon (Forebrain), which is divided into two hemispheres, each containing an independent cavity—the lateral ventricle. Clusters of Nerve Cells form not only the corpora striata at the floor of the lateral hemispheres but also a thin layer in the roof of the hemispheres—the primordial brain roof, or archipallium. The olfactory lobes are weakly demarcated from the hemispheres. The Diencephalon (intermediate brain) is only slightly covered from above by adjacent regions. The epiphysis (Pineal Gland) is situated on top of it. Extending from the floor of the diencephalon is the infundibulum, to which a well-developed Pituitary Gland (hypophysis) is attached. The mesencephalon (Midbrain) is smaller than that of teleost fishes. The Cerebellum is small, appearing as a small ridge lying behind the midbrain at the anterior margin of the Rhomboid fossa—the cavity of the Fourth ventricle (Fig. 159).

Fig. 159. Brain of a frog

Sense Organs enable amphibians to orient themselves both in water and on land. For larvae and aquatic adults, the lateral line system, Olfaction, thermoreception, taste, Hearing, and vision play a vital role. For terrestrial amphibian species, vision is the primary sensory modality for orientation. Tactile corpuscles (clusters of sensory cells innervated by nerve fibers) are scattered across the eye. Visual organs are well-developed in most terrestrial amphibians (Fig. 160).

Fig. 160. Structure of a frog's eye

Compared to fish, the auditory organ in amphibians has become more complex (Fig. 161).

Fig. 161. Head of a frog with open mouth:

1 - tongue; 2 - choanae; 3 - Eustachian tubes; 4 - glottis; 5 - vomerine teeth; 6 - tympanic membrane; 7 — eyes

Subterranean amphibians (caecilians) and those inhabiting underground waters (such as the European proteus, the blind salamander Typhlotriton spelaeus, and a few other species) have tiny eyes that are barely visible through the skin or entirely hidden. Larval eyes lack eyelids. During metamorphosis, movable upper and lower eyelids develop, along with a nictitating membrane that separates from the lower eyelid. The retina contains rods and cones, with rods predominating in crepuscular and nocturnal species. Color Vision is well-developed in many amphibians.

The membranous labyrinth of amphibians, housed within the Inner ear capsule, has undergone little change. A new region has emerged—the middle ear, which contains an apparatus that amplifies sound wave perception. The middle ear cavity originated from the rudiment of the gill cleft located between the jaws and the hyoid arch. The outer opening of the middle ear is covered by a thin, elastic membrane. Located within the cavity is a rod-like

ossicle, the stapes, which abuts the tympanic membrane with one end and the oval window—covered by a smaller membrane—with the other.