Vertebrate Zoology: A Study Guide - T. A. Dauda 2014

Multicellular Animals
Fishes
Class Amphibia

General characteristics

The Class Amphibia comprises about 2,600 known animal species whose larvae develop in an aquatic environment, while adult forms can inhabit both Water and land. Amphibians are distributed globally, though they are most abundant in regions with warm, humid climates. Their appearance varies significantly (Fig. 19). The body may be elongated (in urodeles), shortened and flattened (in anurans), or worm-like (in apodans). The HEAD is movably connected to the trunk in amphibians, although the neck region is not externally distinct.

Fig. 20 Frog Skeleton:

A — general view; B — vertebra from above; C — vertebra from the front; 1 — cervical vertebra; 2 — sacral vertebra; 3 — urostyle; 4, 5 — Sternum; 6 — presternum; 7 — coracoid; 8 — procoracoid; 9, 10 — scapula; 11 — ilium; 12 — ischium; 13 — pubis; 14 — humerus; 15 — BONES OF THE forearm; 16 — carpus; 17 — metacarpus; 18-20 — Phalanges of the fingers; 21 — Femur; 22 — bones of the lower leg; 23 — tarsus; 24 — metatarsus; 25 — calcar (spur); 26 — phalanges of the toes of the hind limb; 27 — vertebral centrum; 28 — spinal canal; 29 — articular facet; 30 — neural spine; 31 — transverse process.

Structural Features of Amphibians

The Skin of amphibians lacks scales; it is thin, naked, moist, and abundant in various glands. The secretions of these cutaneous glands ensure a continuous fluid film on the body surface, which prevents desiccation and is essential for gas exchange during cutaneous Respiration. The secretion of certain glands possesses bactericidal properties, preventing pathogenic microorganisms from penetrating the skin. Poison gland secretions protect amphibians from predators. The coloration of amphibian integument is typically protective, matching the Background of their natural habitat. Some species, such as green tree frogs, can change their color to match the surface they rest on.

The skeleton of amphibians consists of the Skull, Vertebral Column, limb bones, and their girdles (Fig. 20). The braincase remains cartilaginous throughout the animal's life. There are relatively few bones in the skull. The visceral part of the skull also remains largely cartilaginous. Among the bones present in this region, the paired bones of the upper and lower jaws are the most developed. Frogs possess Teeth on the upper jaw, whereas toads lack them entirely.

THE VERTEBRAL COLUMN of amphibians is more differentiated than that of fish. It is divided into cervical, trunk, sacral, and caudal regions. The cervical region consists of a single vertebra, which articulates with the skull via two condyles located on the vertebral body.

The trunk region in amphibians contains A large number of vertebrae, ranging from 7 (in anurans) to 100 (in apodans). Furthermore, anurans lack Ribs, whereas apodans possess short ones. There is a single sacral vertebra featuring long processes that articulate with the iliac bones of the Pelvic Girdle. The caudal region is well-developed only in urodeles; it is reduced in apodans and represented solely by a single bone (the urostyle) in anurans.

Due to the absence of ribs and a sternum, the Pectoral Girdle of amphibians lies freely within the Muscle mass. Its upper part is represented by the scapula, and the lower part by the coracoid. Anurans also possess a slender, rod-like clavicle.

The pelvic girdle is formed by three paired bones: the ilium, pubis, and ischium, which connect at their ends to form the acetabulum.

The Skeleton of the paired pentadactyl limb typically comprises three segments: the upper arm (humerus), forearm (radius and ulna), and hand (carpals, metacarpals, and phalanges).

The skeleton of the hind limb includes three segments: the thigh (femur), lower leg (Tibia and Fibula), and FOOT (tarsals, metatarsals, and phalanges). In anurans, due to the fusion of the radius and ulna, as well as the tibia and fibula, these segments are represented by fewer bones than in the typical case described above.

Fig. 21 Internal Structure of a frog:

1 — Heart; 2 — lung; 3, 4 — Liver; 5 — gall bladder; 6 — Stomach; 7 — Pancreas; 8, 9 — Small Intestine; 10 — Large Intestine; 11 — Spleen; 12 — cloaca; 13 — Urinary Bladder; 14 — opening of the bladder into the cloaca; 15 — Kidney; 16 — Ureter; 17 — opening of the ureter into the cloaca; 18 — Ovary; 19 — fat body; 20, 21 — oviducts; 22 — uterine region of the oviduct; 23 — opening of the oviduct into the cloaca; 24 — dorsal aorta; 25 — posterior vena cava; 26 — carotid artery; 27 — aortic arch; 28 — pulmonary artery.

The musculature of amphibians is more differentiated than that of fish. Compared to fish, only a portion of the trunk musculature in amphibians retains a segmented, ribbon-like structure. Amphibians also develop specialized Muscles. A frog, for example, has over 350 muscles, with the largest and most powerful ones associated with the free limbs.

The Digestive System begins with a wide Mouth opening that leads into a spacious oral-pharyngeal cavity (Fig. 21). The ducts of the Salivary Glands, the glottis, the internal nostrils (choanae), and the openings of the Eustachian tubes—which connect the Pharynx to the Middle ear cavity—all open into this space. A true Tongue with its own musculature is located on the floor of the oral-pharyngeal cavity. In frogs, it is attached to the anterior part of the lower jaw and can be flicked out of the mouth using its free end, which rests pointing backward when at rest. In all amphibians, the tongue secretes a sticky substance used for catching small insects.

Teeth are not present in all species. The eyes assist in pushing the food bolus from the oral-pharyngeal region into the Esophagus. By contracting their muscles, amphibians retract their eyeballs deep into the Oral Cavity, where they exert pressure on the food and push it forward.

The short esophagus empties into a poorly defined stomach. Overall, the intestine in amphibians is longer than in fish and is subdivided into the small, large, and rectum. The small intestine receives pancreatic secretions and Bile produced by the liver. There is no sharp boundary between the small and large intestines in amphibians. The rectum is well-defined and opens into the cloaca, which also receives the openings of the reproductive ducts and the urinary bladder duct.

Respiration in adult amphibians involves both the Lungs and the skin. The lungs are paired, thin-walled sacs with honeycomb-like internal walls. Because the lungs are relatively simple and inefficient, cutaneous respiration plays a crucial role in amphibians. In the green frog, for instance, about 51% of oxygen is absorbed through the skin. Amphibian larvae breathe using branched external gills, which disappear in the vast majority of species during metamorphosis into the adult stage. The Mechanism of pulmonary respiration is unique: the oral-pharyngeal cavity acts as a pump, the floor of which alternately drops (drawing in air through open nostrils) and rises (forcing air into the lungs while the nostrils are closed).

The Circulatory system OF amphibians is more complex than that of fish. Due to air-breathing, amphibians possess two circuits of Blood Circulation. The Heart is three-chambered, consisting of two atria and a single ventricle. The atria communicate with the ventricle through a common opening. A truncus arteriosus extends from the ventricle, distributing blood into the arterial vessels, which carry a mixture of mixed, arterial, and venous blood to various PARTS OF THE body. Blood returns to the heart via Veins that expand to form a sinus venosus, which opens into the right atrium.

Compared to fish, The Nervous system of amphibians exhibits several progressive features. Notably, the Brain is characterized by an enlarged Forebrain divided into two hemispheres. At the same time, the Midbrain is relatively small, and the Cerebellum is very rudimentary. The weak development of this brain region is correlated with the relatively simple, stereotyped movements of amphibians (Fig. 22).

Fig. 22. Frog brain:

A — dorsal view; B — ventral view; C — lateral view; 1 — cerebral hemispheres;

2 — olfactory lobes; 3 — Olfactory nerve; 4 — Diencephalon; 5 — optic nerves; 6 — infundibulum of the diencephalon; 7 — Pituitary Gland; 8 — midbrain; 9 — cerebellum; 10 — Medulla Oblongata; 11 — Spinal Cord.

Due to the peculiarities of Vision in a gaseous medium, the visual Organs of amphibians feature a convex cornea, a biconvex lens, and movable eyelids that protect the eyes from drying out and contamination. In addition to the upper and lower eyelids, amphibians also possess a third eyelid, or nictitating membrane, located in the anterior corner of the eye.

The auditory organs of amphibians are more complex than those of Fishes, comprising both inner and middle ears. The middle ear communicates with the pharynx via the Eustachian tube and is separated from the external environment by the tympanic membrane.

The olfactory organs are represented by paired olfactory capsules that communicate with the external environment through the external nostrils. From these capsules, internal nostrils (choanae) extend and open into the oro-pharyngeal cavity.

The lateral line system is well-developed in the larvae of all amphibians. In adults, it persists only in aquatic tailed amphibians and a few aquatic tailless ones. The sensory Cells of this organ are located superficially in the skin rather than within a recessed canal, as is the case in fishes.

The excretory organs of adult amphibians are represented by a pair of trunk mesonephric Kidneys. Each kidney gives rise to a single ureter. Urine eliminated through these Ureters first enters the cloaca and then the urinary bladder. Upon filling the bladder, the urine passes back into the cloaca and is subsequently expelled.

The reproductive organs of amphibians are relatively simple in structure. Paired Testes are located close to the kidneys. Their efferent ducts pass through the upper region of the kidneys and empty into the ureters, through which both urine and male Gametes are thus discharged. Large paired Ovaries in females lie within the body cavity. Mature eggs are initially released into the body cavity and then enter the funnel-shaped opening of the oviducts. As they pass through the oviducts, the eggs become coated with a thick, transparent gelatinous layer. The oviducts open into the cloaca.

Fig. 23. Development of the frog:

1 — spawn attached to plants (individual eggs shown separately); 2-7 — stages of tadpole development; 8, 9 — metamorphosis of the tadpole into a frog; 4a — head of a tadpole with external gills; 4b — gills.

Females of most anuran amphibian species spawn eggs into the water, where they are fertilized by males. In contrast, tailed amphibians may exhibit internal Fertilization. For instance, a male newt deposits packets of sperm enclosed in a mucous pouch—spermatophores—onto aquatic plants. The female finds the spermatophore, captures it with the edges of her cloaca, strips away the envelope, and draws it inside.

Amphibian development proceeds via metamorphosis (Fig. 23). The eggs hatch into larvae, or tadpoles, which are strictly aquatic animals. They respire via gills, possess a circulatory system similar to that of fish, and retain lateral line organs. They move through the water primarily by undulating their flattened, fin-rimmed tails. During development, the larva undergoes metamorphosis: land-type paired limbs appear; gill breathing is replaced by pulmonary respiration; two circulation loops (SYSTEMIC AND PULMONARY) develop; and in tailless amphibians, the tail is resorbed.

The class Amphibia includes three orders: Urodela (tailed amphibians), Anura (tailless amphibians), and Apoda (caecilians).