Question

The Class Amphibia is divided in three living orders: Anura, Urodela and Gymnophiona. Provide a detailed account of 1) the typical life- cycle of an amphibian and 2) some specific patterns distinct to each order. You should make reference to reproduction, development and behaviour to structure your answer.

Answer 1

Amphibia is the class of tetrapoda which have aquatic larval stage and terrestrial adult stage. They were the first tetrapods to evolve during devonian. Tetrapoda is the superclass of four - footed animals where the limbs are pentadactyl or with 5 digits.

CHARACTERISTICS:

Amphibia includes anamniotes ( without amnion) , poikilothermal or ectothermal animals ( cold blooded in which temperature of the body varies according to that of surroundings) which live both in fresh water and moist soil. Marine forms are absent.

Larval stage is fish - like which is aquatic. It has a tail for swimming. Adults are morphologically different from larvae. They usually live on land. Breeding requires water environment. Therefore, amphibians though adapted to terrestrial life, have to be near water for completing their life cycle.

Body is divided into head and trunk and neck is absent. Eyes have eyelids. Middle ear is present. Tympanum lies on the outside to receive vibrations.

Reproductive, urinary, and digestive tracts open into common cloacal chamber or cloaca. It leads to exterior through a cloacal aperture.

Skin is naked without scales.It has glands and is kept moist.

The animals have two pairs of pentadactyl limbs. They are used for walking and swimming. Fore limbs are generally end in four digits while hind limbs have five digits without scales. Digits do not have claws or nails. They may be webbed.

Heart is three chambered. RBCs are nucleated.

Gills occur in larval stage which may also posses unpaired fins. Gills occurs in some adults in addition to lungs. Respiration can also occurs through skin and buccal cavity.

Cranial nerves are 10 pairs.

Nitrogenous excretory product is usually urea. Kidney is mesonephric but the tedpole has pronephric one.

Skull is dicondylic.

Vertebrae are procoelous.

Sexes are separate. Fertilisation is external. Larva undergoes metamorphosis.

Amphibia originated during devonian from stegocephalia like animals. Carboniferous is the age of amphibians.

Amphibia is usually divisible into three orders------ apoda, anura and urodela.

ORDER APODA ( Caecilia, Gymnophiona ) :

limbs, girdles, neck , middle ear and vocal cords are absent. Tail is nearly absent. Head bears sensory tentacles.Eyes are functionless. Therefore, caecilians are also called blindworms. Dermal scales are embedded in skin.

Example: Ichthyophis, Uraeotyphlus

ORDER URODELA ( Caudata ) :

Animals have head, neck , trunk and tail. The tail may have caudal fin without fin rays. Skin is smooth. Middle ear and vocal cords are absent but eye lids occur. Gills and gill slits may occur even in the adult.

Examples: Necturus, salamandra, Ambystoma, Amphiuma, proteus, Cryptobranches.

ORDER ANURA ( salientia) :

The order includes common amphibians, Frogs and toad.Body is divided into head and trunk. Forelimbs are short. Hind limbs are long. Webbed digits are present for swimming. Skin is smooth in frogs and watery in toads. Vocal cord develops for the first time.middle ear is present. Fertilisation is external. Larval stage has gills and tail. Examples: Rana , Bufo, Hyla, Alytes, Pipa and Bombinator.