Vertebrate Zoology - V. M. Konstantinov 2011

Chordates
Terrestrial, or Tetrapod, Vertebrates
Class Amphibians, or Amphibia

General characteristics

Amphibians are a relatively small group of the most primitive terrestrial vertebrates. As a whole, the group exhibits transitional features between an aquatic and a terrestrial lifestyle (as reflected in their name). The overwhelming majority of amphibians inhabit either Water or land depending on their life cycle stage. Throughout their lives, they typically undergo metamorphosis, transforming from purely aquatic larvae into adult forms that live mostly out of water. Consequently, their Respiratory system shifts from gill breathing to lung breathing, accompanied by corresponding Changes in the Circulatory system. In The Musculoskeletal System, pentadactyl limbs develop, and the sensory system undergoes significant modification. However, the degree of adaptation to terrestrial life in adult forms is generally modest.

The total number of extant amphibian species is approximately 4,500. They are classified into three orders: Caudata (or Urodela), Apoda, and Anura (or Ecaudata).

Anatomy of Amphibians

Integument. The Skin of all amphibians is smooth and devoid of scales. The multilayered epidermis is rich in multicellular mucous glands (Fig. 68) embedded in the corium. The Functions of the Skin glands are diverse. They produce a liquid film on the skin surface, which is essential for gas exchange. This film also provides some protection against desiccation. The mucus AIDS in the coagulation of foreign particles on the body surface, exhibits bactericidal properties, and protects the body from the invasion of pathogenic microbes through the skin. Poisonous skin glands provide amphibians with significant protection against predators. Venomous amphibians often display bright coloration, warning predators of danger before striking prey. It has been established that in some anuran amphibians, the upper layer of the epidermis becomes keratinized. This is most pronounced in toads, where the horny layer on the back makes up approximately 60% of the total epidermal surface. In most amphibians, weak keratinization of the epidermis does not prevent water from penetrating through the skin into the underlying lymphatic sacs.

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Fig. 68. Cross-section of salamander skin:

1 — gland secretion protruding to the exterior; 2 — pigment layer; 3 — mucous skin glands; 4 — poison skin gland; 5 — severed Blood Vessels; 6 — epidermis; 7 — fibrous layer of the skin

Skeleton. Compared to the fish skeleton, the amphibian skeleton exhibits A number of modifications. In the Axial Skeleton, THE Vertebral Column of amphibians is more distinctly segmented due to their semi-terrestrial lifestyle. It includes cervical, trunk, sacral, and caudal regions (Fig. 69). The cervical region is represented by a single vertebra; its centrum is small and bears two articular facets, through which the vertebra articulates with the Skull. The number of trunk vertebrae varies: the lowest is in anurans (usually 7), and the highest is in caecilians (over 100). The single sacral vertebra (absent in caecilians) bears long transverse processes that articulate with the iliac BONES OF THE Pelvic Girdle. The caudal region is well developed in urodeles, very small in caecilians, and represented by a single bone—the urostyle—in anurans; during embryonic development, it forms as a series of separate vertebrae whose centra subsequently fuse.

Fig. 69. Frog Vertebral Column and pelvic girdle, dorsal view:

I — cervical region (consisting of a single vertebra); II — trunk region; III — sacrum; IV — urostyle; 1 — neural spine of the third trunk vertebra; 2 — transverse process of the same vertebra; 3 — articular surfaces on the first cervical vertebra

The shape of the vertebrae varies extensively across different Representatives of the Class Amphibia. In lower amphibians (caecilians, lower urodeles), they are amphicoelous, meaning the notochord persists throughout life between the vertebrae. In anurans, the vertebrae are procoelous, being concave anteriorly and convex posteriorly; in higher urodeles, they are opisthocoelous, i.e., convex anteriorly and concave posteriorly. There are numerous exceptions to this general description; for instance, the primitive New Zealand frog Leiopelma possesses amphicoelous vertebrae. True Ribs are absent in anuran amphibians, whereas very short ribs are present in caecilians; urodeles develop short "upper" ribs.

Braincase. The chondrocranium remains largely cartilaginous throughout life (Fig. 70). This is due to the weak development of endochondral and dermal ossifications. The following endochondral bones develop in the primary braincase. In the occipital region, there are only two lateral occipital bones; the areas corresponding to the basisphenoid and supraoccipital bones of fish remain cartilaginous. In the region of the auditory capsule, a single small otic bone forms, while the greater part of the capsule remains cartilaginous. In the anterior part of the Orbit, anurans develop a single sphenethmoid bone, whereas in urodeles, this bone is paired. The olfactory capsule is cartilaginous.

Dermal bones are also few in number. The roof of the skull is formed by the parietal and frontal bones, which fuse into frontoparietals in anurans. Anterior to them lie the nasal bones, which fuse with the premaxillary bones in caecilians. On the sides of the posterior part of the skull are squamosal bones, which are particularly well developed in caecilians. The floor of the skull is paved by a large parasphenoid, anterior to which lie paired vomerine bones.

The floor of the skull also incorporates bones of the visceral skeleton—the palatines and pterygoids. The former adjoin the vomers, while the latter adjoin the squamosal bones. They develop on the ventral surface of the palatocartilage. The functions of the upper jaws are performed, as in bony fish, by the premaxillary (or intermaxillary) and maxillary bones.

Fig. 70. SKULL OF A frog:

A — dorsal view; B — ventral view (dermal bones removed from one side). Bone designations: 1 — exoccipital; 2 — frontoparietal; 3 — nasal; 4 — premaxillary; 5 — prootic; 6 — squamosal; 7 — parasphenoid (left half); 8 — Vomer; 9 — palatine; 10 — pterygoid; 11 — sphenethmoid; 12 — maxillary; 13 — quadratojugal; 14 — Optic nerve foramen; 15 — Trigeminal nerve foramen

The lower jaw is represented by Meckel's Cartilage, which is covered externally by the dentary and angular bones.

The amphibian skull is autostylic, meaning the palatocartilage is directly fused to the braincase. Due to this autostyly, the hyoid arch does not participate in attaching the jaw apparatus to the skull.

The upper element of this arch—the hyomandibula—is transformed into a small bone, the stapes (columella), whose proximal end rests against the auditory capsule and whose outer (distal) end abuts the tympanic membrane. With The formation of the Middle ear cavity, this bone lies within the cavity and functions as an auditory ossicle. Thus, the hyomandibula originates from the System of the fourth (hyoid) visceral arch (see Fig. 70).

The lower elements of the hyoid arch and gill arches are modified into the hyoid plate and its horns. This plate is situated between the rami of the lower jaw. Its anterior horns curve upward, flank the gut laterally, and attach to the auditory capsules. Changes in the visceral skeleton are accompanied by the loss of opercular folds.

Thus, the amphibian skull differs from that of most bony fish in: 1) the weak development of endochondral and dermal ossifications; 2) autostyly; 3) the Modification of the hyoid and gill arches, which are transformed partly into the auditory apparatus and partly into the hyoid apparatus; 4) the reduction of the operculum.

Girdles of the limbs. The Pectoral Girdle has the shape of an arch with its apex directed toward the ventral surface of the animal (Fig. 71). Each half of the arch (left and right) consists of the following main elements. The upper (dorsal) part is represented by the scapula with a broad suprascapular cartilage. The lower (ventral) part includes the coracoid and the procoracoid lying anterior to it. In anurans, a slender, rod-like clavicle is located between the episternum and the scapula. These girdle elements converge at the point of attachment of the humerus to form the glenoid cavity. Anterior to the junction of the left and right coracoids lies the episternum, and posteriorly lies the Sternum. Both of these bones terminate in cartilage. Unlike in bony fish, the pectoral girdle lies freely within the musculature and is not connected to the skull. Due to the absence or incomplete development of ribs, a true rib cage is lacking in amphibians.

The pelvic girdle (Fig. 72) is formed by three paired elements converging in the acetabulum, which they collectively form. The long iliac bones are attached by their proximal (anterior) ends to the transverse processes of the single sacral vertebra. Directed forward and downward, the pubic element of the girdle remains cartilaginous in frogs. Posterior to it lies the ischium. This arrangement of the pelvic girdle elements is characteristic of all tetrapods.

Skeleton of the free limbs. The skeleton of the free limbs is typical for terrestrial vertebrates and differs significantly from that of fish. While fish fins structurally represent simple single-segment levers that move only relative to the body and lack intrinsic musculature, the limbs of tetrapods are multi-segmented levers powered by fairly strong Muscles. In this case, not only does the entire limb move relative to the body, but individual limb segments also move relative to one another.

Fig. 71. Pectoral girdle of a frog, anterior view:

1 — sternum; 2 — cartilaginous anterior and posterior PARTS OF THE sternum; 3 — coracoid; 4 — clavicle lying on the procoracoid; 5 — presternum; 6 — scapula; 7 — glenoid cavity on the scapula for the humerus (cartilage covered with dots)

Fig. 72. Skeleton of a frog:

A — general view; B — vertebra, dorsal view; C — vertebra, anterior view; 1 — cervical vertebra; 2 — sacral vertebra; 3 — urostyle; 4 — sternum; 5 — cartilaginous posterior part of the sternum; 6 — presternum; 7 — coracoid; 8 — procoracoid; 9 — scapula; 10 — suprascapular cartilage; 11 — ilium; 12 — ischium; 13 — pubic cartilage; 14 — humerus; 15 — forearm (radius + ulna); 16 — carpus; 17 — metacarpus; 18 — vestigial digit I; 19 — digit II; 20 — digit V; 21 — Femur; 22 — shank (Tibia and Fibula); 23 — tarsus; 24 — metatarsus; 25 — rudimentary extra digit; 26 — digit I; 27 — vertebral centrum; 28 — neural canal; 29 — articular facet; 30 — neural spine; 31 — transverse process

In general Structure, the pentadactyl limb skeleton consists of several main sections. Specifically, in amphibians, the skeleton of both the forelimbs and hindlimbs comprises the following divisions (see Fig. 72):

I — upper arm in the forelimb, thigh in the hindlimb; this section always consists of a single bone attached proximally to the girdle;

II — forearm in the forelimb, shank in the hindlimb. Typically, this section consists of two parallel bones: the ulna and radius in the forearm, and the tibia and fibula in the shank;

III — manus (hand) in the forelimb and pes (FOOT) in the hindlimb; this section consists of three subsections:

1) carpus in the forelimb, tarsus in the hindlimb; this subsection is typically represented by 9–10 small bones arranged in three rows;

2) metacarpus in the forelimb, metatarsus in the hindlimb; typically, this subsection consists of 5 elongated bones arranged in a single row, fan-like, extending from the carpus or tarsus;

3) Phalanges of the four to five digits, which represent a continuation of the metacarpus or metatarsus and comprise three to five rows of small bones each.

The limb skeleton of urodele amphibians closely follows this general pattern. In anurans (frogs), the following modifications have occurred: both elements of the forearm and shank have fused into single bones, most of the Carpal Bones (up to 6) and Tarsal Bones (up to 3) have fused together, and a rudimentary extra digit is present preaxial to the first digit of the hindlimb. These features are secondary in nature and are apparently associated with the adaptation of frogs to saltatorial (jumping) locomotion.

Muscular system. The muscular system of amphibians differs significantly from that of fish in two main aspects, which are related to locomotion via pentadactyl limbs and, to some extent, movement across a solid substrate. First, a powerful and complexly organized musculature develops on the free limbs. Second, in connection with complex movements, the body musculature is more differentiated, and the segmentation characteristic of fish is disrupted in amphibians. Myamerism of the muscular system is more pronounced in urodeles and apodans. Conversely, in anurans, it can be observed only in a few Regions of the trunk in adults and in the larval stage.

Digestive Organs. The oral gape leads into a spacious buccopharyngeal cavity, which narrows into the Esophagus. The internal nares (choanae), Eustachian tubes (openings of the middle ear cavities), and the glottis open into the buccopharyngeal cavity. The ducts of the Salivary Glands, which are absent in fish, also open here. Their secretion serves to moisten the food bolus and has no chemical effect on the food. A true muscular Tongue is located on the floor of the buccopharyngeal cavity. The shape of the tongue varies. In some urodeles, it is attached immovably; in others, it is mushroom-shaped, sitting on a slender stalk. In frogs, the tongue is attached by its anterior end to the floor of the Mouth, while its free portion points backward inside the mouth when at rest. In all amphibians, the tongue secretes a sticky substance used for capturing small prey. A few amphibians lack a tongue entirely.

Teeth are present on the premaxillae and maxillae, the vomer, and in some species, on the lower jaw. They appear as small, uniform cones with tips slightly curved backward. Certain species, such as toads, lack teeth on their jawbones. During swallowing, the propulsion of the food bolus from the buccopharyngeal cavity into the esophagus is assisted by the eyeballs, which are separated from this cavity only by a thin mucous membrane and can be retracted somewhat into the buccopharynx with the help of specialized muscles.

The short esophagus empties into a relatively poorly differentiated Stomach. The intestine proper is relatively longer than that of fish. The Pancreas lies within the loop of the anterior (small) intestine. The large Liver possesses a gall bladder, and its Bile duct empties into the anterior part of the Small Intestine (the duodenum). The pancreatic ducts also empty into the bile duct, as the pancreas lacks an independent opening into the intestine (Fig. 73). The second section of the intestine—the Large Intestine—is indistinctly demarcated from the small intestine. In contrast, the third, rectal section is well defined and terminates in the cloaca.

Respiratory organs. The respiratory organs of amphibians are diverse. In the adult stage, most species breathe via Lungs and through the skin. The lungs are paired sacs with thin, alveolar walls. Given the relatively small surface area of the lungs, cutaneous (skin) respiration is of great importance. The ratio of lung surface area to skin surface area in amphibians is 2:3 (whereas in mammals, the internal surface area of the lungs is 50–100 times greater than the skin surface). In the edible frog, 51% of oxygen is absorbed through the skin. The skin also plays a vital role in eliminating carbon dioxide from the body: 86% of it is excreted through the skin and 14% through the lungs.

Fig. 73. General Anatomy of the viscera in a female frog:

1 — esophagus; 2 — stomach; 3 — liver lobes; 4 — pancreas; 5 — small intestine; 6 — large intestine (rectum); 7 — cloaca (opened); 8 — ventricle of The Heart; 9 — left atrium; 10 — right atrium; 11 — carotid artery (right); 12 — left aortic arch; 13 — pulmocutaneous arch (right); 14 — posterior (inferior) vena cava; 15 — abdominal vein; 16 — lungs; 17 — left Kidney (posterior end); 18 — right Ovary; 19 — left oviduct; 20 — its opening (ostium); 21 — Urinary Bladder; 22 — gall bladder; 23 — Spleen; 24 — anterior vena cava (right)

Table 6

Dependence of O₂ consumption and CO₂ output on ambient Temperature in the green frog (Rana)

(after L. Prosser, 1977)

Cutaneous respiration is of great functional importance not only due to the imperfection of the lungs, but also as an adaptation ensuring blood oxygenation during prolonged immersion in water, such as during hibernation or when hiding in a body of water to evade terrestrial predators. In these instances, respiration occurs exclusively through the skin, and the right atrium (into which oxygenated blood from the cutaneous vein empties via the anterior venae cavae — see above) receives oxygen-rich blood.

In American lungless salamanders and the Far Eastern newt, the lungs are completely atrophied, and gas exchange takes place entirely through the skin and the mucous membrane of the Oral Cavity.

The capacity of the skin and lungs to absorb oxygen (O₂) and release carbon dioxide (CO₂) in amphibians depends on the ambient temperature (Table 6). As shown, at an ambient temperature of 5 °C, 1.5 times more oxygen enters through the skin of the green frog than through the lungs. At 25 °C, conversely, twice as much oxygen enters through the lungs as through the skin. The situation with CO₂ elimination is different. When the temperature rises from 5 to 25 °C, The Role of the skin in carbon dioxide output increases by only 3.3 times, whereas that of the lungs increases 7-fold. At all temperatures, THE CONTRIBUTION OF the skin to the removal of CO₂ from the frog's body is noticeably greater than that of the lungs.

Amphibian larvae respire using branching external gills, which subsequently disappear in the vast majority of species, but persist throughout life in proteuses and sirens. Adult amphiumas possess internal gills alongside lungs.

Due to the absence of a rib cage, The Mechanism of pulmonary respiration is highly peculiar. The buccopharyngeal cavity acts as a force pump: its floor alternately lowers (drawing in air while the nostrils are open) and rises (forcing air into the lungs while the nostrils are closed). Consequently, the amphibian skull is exceptionally broad while being shallow; the efficiency of pulmonary respiration increases with the distance between the rami of the lower jaw. For example, it reaches its maximum width in toads, whose keratinized skin bears a low respiratory load.

Fig. 74. Diagram of the dissected frog heart:

1 — right atrium; 2 — left atrium; 3 — ventricle; 4 — valve closing the common opening leading from both atria into the ventricle; 5 — conus arteriosus; 6 — truncus arteriosus; 7 — pulmocutaneous artery; 8 — aortic arch; 9 — common carotid artery; 10 — carotid gland; 11 — spiral valve of the conus arteriosus

Circulatory system. The heart in all amphibians is three-chambered, consisting of two atria and a single ventricle (Fig. 74). In primitive forms (apodans and urodeles), the left and right atria are not completely separated. In anurans, the interatrial septum is complete, but in all amphibians, both atria communicate with the ventricle via a single common opening. In addition to these main heart chambers, There is a sinus venosus. It receives venous blood and communicates with the right atrium. The conus arteriosus adjoins the heart, receiving blood ejected from the ventricle. The conus arteriosus contains a spiral fold that assists in distributing blood into the three pairs of vessels emerging from it. The cardiac index (the ratio of heart mass to body mass expressed as a percentage) varies among different species and depends on the animal's locomotor activity. For instance, in the relatively sedentary marsh and green frogs, it is 0.35 – 0.55%, whereas in the strictly terrestrial (except during the breeding season) and active green toad, it reaches 0.99%.

In higher amphibians — anurans — three pairs of arterial arches originate from the conus arteriosus.

The first pair (counting from HEAD to tail) carries blood to the head and represents the carotid Arteries. They are homologous to the first pair of fish branchial arches. The second pair, which also departs from the ventral side of the conus arteriosus, is homologous to the second pair of fish branchial vessels and is termed the systemic aortic arches. They give rise to the subclavian arteries, which supply blood to the pectoral girdle and forelimbs. The right and left systemic arches curve around to join together, forming the dorsal aorta situated beneath the vertebral column; arteries branching from it supply the Internal Organs. The final, third pair, homologous to the fourth pair of fish branchial arches, arises not from the ventral but from the DORSAL SIDE OF the conus arteriosus. It delivers blood to the lungs and constitutes the pulmonary arteries. Each pulmonary artery gives off a vessel carrying venous blood to the skin: these are the cutaneous arteries.

In tailed amphibians possessing lungs, the layout of the arterial vessels is fundamentally similar. However, unlike anurans, they retain a pair of arteries corresponding to the third pair of branchial vessels; thus, the total number of paired arterial trunks is four rather than three as in anurans. Furthermore, the pulmonary arteries maintain a connection with the systemic aortic arches via the so-called ductus arteriosus (botallique ducts) (see Fig. 77).

In tailed amphibians that retain gills throughout life, the circulatory pattern closely resembles that of fish and larvae of higher amphibians. Four pairs of arterial arches arise from the ventral aorta. The carotid arteries branch off from the efferent part of the first arch, and the carotid ducts are retained. With the appearance of lungs, pulmonary arteries develop, originating from the fourth arterial arch. The circulatory scheme in this case is almost identical to that of lungfishes (see Fig. 44).

The Venous system of primitive amphibians resembles that of lungfishes. The caudal vein divides into two renal portal Veins. Blood from the Kidneys passes into the unpaired posterior vena cava and the paired posterior cardinal veins. At the level of the heart, the latter fuse with the paired jugular, subclavian, and cutaneous veins to form the ducts of Cuvier, which empty into the sinus venosus. Blood from the intestine is collected by the subintestinal and abdominal veins, which unite to form the HEPATIC PORTAL VEIN. Blood flows from the liver into the vena cava via the hepatic vein.

In anurans, the cardinal veins are not retained, and all blood from the trunk region is ultimately collected into the posterior vena cava, which empties into the sinus venosus. As in the previous case, abdominal and subintestinal veins are present, forming the hepatic portal system. Due to the absence of cardinal veins, anurans do not form the ducts of Cuvier. Instead, the jugular veins unite with the subclavians to form paired anterior venae cavae (superior venae cavae), which open into the sinus venosus. The cutaneous VEINS OF THE corresponding side also empty into the superior venae cavae, carrying arterial rather than venous blood.

The Pulmonary veins transport blood directly into the left atrium. Venous blood (with a quite substantial admixture of oxygenated blood entering from the cutaneous veins into the anterior venae cavae) pours into the sinus venosus, and from there into the right atrium. Upon atrial contraction, both venous and arterial blood flows through the common ventriculoatrial opening into the ventricle. The conus arteriosus adjoins the right side of the ventricle, initially receiving the most venous blood, which is then directed into the open apertures of the pulmo-cutaneous arteries. The openings of the other arterial arches are closed at this time by the spiral valve of the conus arteriosus. As ventricular contraction continues, pressure within the conus arteriosus rises, the spiral valve shifts, and the openings of the systemic aortic arches are uncovered, through which mixed blood from the central part of the ventricle enters. Further Displacement of the spiral valve opens the ostia of the carotid arteries, allowing the most oxygenated blood — which exits the conus arteriosus last from the left side of the ventricle — to pass through. Nevertheless, complete Separation of arterial and venous blood does not occur: mixed blood is delivered to the body organs.

Fig. 75. Frog Brain:

dorsal view (A), ventral view (B), lateral view (C), longitudinal section (D); 1 — cerebral hemispheres; 2 — olfactory lobe; 3 — Olfactory nerve; 4 — Diencephalon; 5 — optic chiasm; 6 — infundibulum; 7 — Pituitary Gland (hypophysis); 8 — optic tectum (Midbrain); 9 — Cerebellum; 10 — Medulla Oblongata; 11 — Fourth ventricle; 12 — Spinal Cord; 13 — Third ventricle; 14 — cerebral aqueduct (Sylvian aqueduct); 15 — Cranial Nerves

The blood flow velocity (an important indicator of metabolic intensity) in amphibians is low, as indirectly evidenced by the pulse rate. In a 50 g grass frog, it is 40 – 50 beats per minute. For comparison, in a bird of similar size, this figure is approximately 500. Bradycardia is observed in aquatic forms. Blood pressure levels in amphibians are also low: approximately 22/12 – 30/25 in urodeles, and 30/20 in anurans. For comparison, in squamate reptiles, this value is about 80/60 (L. Prosser, 1978).

Nervous system. The brain (Fig. 75) exhibits a number of progressive features. This is reflected in the relatively larger Forebrain compared to Fishes, the complete separation of its hemispheres, and the fact that not only the floor of the Lateral ventricles but also their sides and roof contain Nerve Cells. Thus, amphibians possess a true cerebral roof — the archipallium — which among bony fishes is found only in lungfishes. The midbrain is relatively small in size. The cerebellum is very diminutive, and in some urodeles (such as proteuses) it is practically indiscernible. The weak development of this brain region is associated with the extremely monotonous and simple movements of amphibians. The final division, the medulla oblongata, is well-developed. Ten pairs of cranial nerves (I–X) emerge from the brain.

Spinal Nerves in both tailed and tailless amphibians form the brachial and lumbar plexuses. The Autonomic nervous system is well developed, represented by nerve trunks located along the sides of the spinal column.

Amphibians have significantly more developed Sense Organs than fish. Their olfactory organs are represented by paired nasal capsules whose internal surface is lined with olfactory epithelium. They communicate with the external environment through paired external nostrils; internal nostrils (choanae) extend from the olfactory capsules and open into the oropharyngeal cavity. As in all terrestrial vertebrates, this system serves for both Olfaction and respiration.

The lateral line system is characteristic of the larvae of all amphibians. In adults, it persists only in aquatic forms of tailed amphibians and a few equally aquatic tailless ones. Unlike in fish, the sensory cells of this organ are not located within a recessed canal, but superficially in the skin.

Taste organs. Located in the oral cavity. It is hypothesized that frogs perceive only bitter and salty tastes.

Visual organs. Amphibian eyes exhibit several features associated with their semi-terrestrial lifestyle: 1) movable eyelids protect the eyes from drying out and contamination; moreover, in addition to the upper and lower eyelids, there is a third eyelid, or nictitating membrane, located in the anterior corner of the eye; 2) a lacrimal gland is present, the secretion of which washes the Eyeball; 3) a convex cornea (rather than flat, as in fish) and a lens-shaped (rather than spherical, as in fish) lens. Both of these latter features result in more long-sighted Vision in amphibians (interestingly, the amphibian cornea flattens when submerged in water); 4) visual accommodation is achieved, as in sharks, by shifting the lens through the action of the ciliary Muscle. Evidently, some amphibians possess Color Vision.

The Organ of Hearing is arranged considerably more complexly than in fish and is adapted for perceiving sound stimuli in an air medium. This is most fully expressed in tailless amphibians. In addition to the Inner ear, represented—just as in fish—by a membranous labyrinth, amphibians also possess a middle ear. The latter is a cavity, one end of which opens into the oropharynx, while the other approaches The surface of the head and is covered by a thin membrane called the tympanic membrane ( eardrum). This cavity forms a bend, the apex of which is located near the membranous labyrinth. The upper part of the cavity from the tympanic membrane to the membranous labyrinth is termed the tympanic cavity. It houses a rod-like bone, the stapes (columella), which abuts the oval window of the inner ear at one end and the tympanic membrane at the other. The lower part of the middle ear cavity, which opens into the oropharynx, is called the Eustachian tube.

Data from comparative anatomy and Embryology show that the middle ear cavity is homologous to the spiracle of cartilaginous fishes, i.e., a vestigial gill slit lying between the mandibular and hyoid arches, while the auditory ossicle is homologous to the upper division of the hyoid arch—the hyomandibula. This example demonstrates that major evolutionary changes in an organ can be achieved through the modification and functional shift of structures previously existing in primitive forms.

In caecilians and urodeles, the tympanic membrane and tympanic cavity are absent, but the auditory ossicle is well developed. The reduction of the middle ear in these groups is apparently a secondary phenomenon.

The lateral line system is characteristic of the larvae of all amphibians. In adults, it persists only in aquatic forms of tailed amphibians and a few equally aquatic tailless ones. Unlike in fish, the sensory cells of this organ are not located within a recessed canal, but superficially in the skin.

Excretory organs. The excretory organs (Fig. 76) are organized similarly to those of cartilaginous fishes. In the embryonic state, the pronephros serves as the excretory organ, whereas in adults, it is the mesonephros with its typical duct system—the Wolffian ducts.

The Ureters open into the cloaca.

In higher terrestrial amphibians, the urinary bladder also opens here. After it fills, urine is discharged through the same opening into the cloaca and then expelled to the exterior.

The number of nephrons in lower (tailed) amphibians is about 500, whereas in higher (tailess) ones, it is about 2,000. Such a noticeable difference is apparently due to the fact that urodele amphibians, which are more closely tied to aquatic environments, also utilize extrarenal excretion (through the skin and gills) of nitrogenous metabolic wastes. Unlike anurans, the nephrons of urodeles (or at least some of them) possess nephrostomes, i.e.,

Fig. 76. Urogenital System of a male frog:

1 — kidneys; 2 — Ureter; 3 — cloacal cavity; 4 — urogenital opening; 5 — urinary bladder; 6 — opening of the urinary bladder; 7 — Testes; 8 — vasa efferentia; 9 — Seminal Vesicle; 10 — fat body; 11 — Adrenal Glands

funnels opening into the body cavity (a primitive feature). Vascular glomeruli within Bowman's capsules are well developed, and amphibians excrete a large volume of hypotonic urine. As an example, in frogs of the genus Rana, The rate of Blood Plasma filtration in the kidneys is about 35 ml/(kg·h).

The primary end product of Protein METABOLISM in amphibians is urea, which has low toxicity but requires a large volume of water for excretion in solution. Physiologically, this is entirely justified, since water uptake by the Organism in amphibians generally presents no difficulty.

The correlation between the type of protein metabolism and environmental conditions is further demonstrated by the following Examples. In the newt during autumn on land, the proportion of ammonia in total nitrogenous waste products is 13%, whereas during its summer aquatic existence, it rises to 26%. In the tadpole, the proportion of ammonia is 75%, whereas in the froglet that has lost its tail and has developed legs, it is only 16%.

Reproductive organs. In males, the paired testes lack independent efferent ducts. The vas deferens passes through the anterior region of the kidney and empties into the Wolffian duct, which thus serves as both a ureter and a sperm duct. Before entering the cloaca, each Wolffian duct in males forms an expansion—the seminal vesicle—which temporarily stores sperm.

Fat bodies—irregularly shaped yellow structures—lie above the testes. They serve to nourish the testes and the spermatozoa developing within them. The size of the fat bodies varies seasonally. In autumn, they are large; in spring, however, during intense Spermatogenesis, their reserves are rapidly consumed, and the size of the fat bodies decreases sharply. Copulatory organs are absent in the vast majority of amphibians (except for caecilians).

Females develop paired Ovaries, with fat bodies also lying above them. Mature eggs enter the body cavity, from which they pass into the funnel-shaped expansions of the paired oviducts—the Müllerian ducts. The oviducts are long, highly convoluted tubes, the posterior sections of which open into the cloaca.

From the foregoing, it is evident that, as in cartilaginous fishes, the urinary and genital ducts in male amphibians are combined into a single Wolffian duct, whereas in females, the Wolffian duct functions exclusively as a ureter, and reproductive products are discharged via an independent genital duct—the oviduct, or Müllerian duct.

Systematics and distribution of modern amphibians

Modern amphibians are represented by two subclasses. Subclass Lepospondyli includes two orders: Caudata (or Urodela) and Apoda (caecilians); subclass Apsidospondyli includes the order Anura (or Ecaudata).