Superclass II Tetrapoda

Amphibia a means dual mode of life. Amphibians can due in water as well as on land.

Class-Amphibia

First cold blooded vertebrates from evolutionary point of view which came to the land. They can live on land as well as in water.

Body varies in form. It may be divisible into head, neck, trunk and tail or only into head and trunk.

There are two pairs of pentadactyl limbs, each with 4-5 or fewer digits. The digits are without claws, nails or hoofs, and often have webs.

Skin is smooth, moist, rich in mucous and poison glands. It is vascular and respiratory in most species. Scales are generally absent.

Endoskeleton is mostly bony. Skull is dicondylic, that is, has two occipital condyles. Most vertebrae are procoelous.

Mouth is large and armed with teeth in the upper or lower or both the jaws. The teeth are acrodont. Vomerine teeth may also occur. Amphibians are the first vertebrates to have a true

tongue. (A true tongue has muscles and is protrusible). Alimentary canal leads into the cloaca.

Respiration takes place by gills, lungs, lining of buccopharyngeal cavity and skin, either separately or in combination. Some forms have vocal cords and vocal sacs.

Heart is 3-chambered: Hepatic portal system and Renal portal system, are well developed. Red blood corpuscles are oval, biconvex and nucleated.

There are 10 pairs of cranial nerves.

The olfactory sacs are paired and dorsal. They communicate with anterior part of the buccopharyngeal cavity by internal nares. Eyes often have movable lids. Middle ear with a single

auditory ossicle. There is no external ear. Lateral line sense organs present in the larvae.

Waste material is removed mostly as urea. Urinary bladder is present. Kidney is mesonephric but the larva has pronephric kidney.

Male usually lacks copulatory organ. Fertilization may be external or internal. Most forms are oviparous.

Some frogs live on trees, e.g. Hyla. Many amphibians show parental care. Male midwife toad (Alytes) carries eggs around their thighs, female surinam toad (Pipa) carries eggs in special

pits on its back till tadpoles become small frogs.

Classification

Three main orders are Apoda or gymnophiona or caecilia, Urodela or caudata and Anura or salientia.

Apoda : Limbless, blind, elongated worm-like; tail short or absent; limb girdles absent; males have protrusible copulatory organs e.g., Ichthyophis, minute scales are embedded in skin.

Ichthyophis : Worm-like, slimy body. Mouth is wide. Eyes are small and covered by opaque skin. There is a sensory tentacle in a groove between nares and eye. Tympanum, limbs and tail are absent. Cloaca is eversible in the male to act as a copulatory organ. Ichthyophis feeds on worms and insects. It lays eggs in wet burrows likely to be filled with rain water. The mother guards the eggs.

Fig. Ichthyophis glutinosa female guarding her eggs

Urodela : Tailed amphibians, tetrapod, gills permanent or lost in adults, males without copulatory organs fertilization external, teeth in both jaws.

e.g., Ambyostoma – American Tiger salamander (Its larva is Axolatl. It shows neoteny i.e. retention of larval characters in adult)

Necturus – Mud puppy or water dog

Proteus – Blind cave salamander

Cryptobranchus – Hell bender

Siren – Mud eel

Amphiuma – Congo eel

Salamandra – Spotted Salamander