Class- Amphibia NEET Special

Class- Amphibia






Devonian origin & carboniferous is the age of amphibians

Class amphibia includes amphibious animals which can live on both the places at ease i.e. under water and on the land. No marine.

These are the first chordate animals which come out of water but these are not able to live on land permanently, these depend on water for their reproduction. Their eggs do not have protecting covering to check the evoporation.

Body is divided into head, trunk and tail. Some amphibians lack tail, e.g. frog, toad etc.

Skin is smooth and mostly scale less, but whenever scales are present there are embeded in the skin e.g. Ichthyophis.

Numerous glands are found in skin which help in moistening the skin. So these animals respire through moist skin. Some poisonous glands are also found in the skin of some animals e.g. Bufo.

Pigment cells are also found as chromatophore for colouration. Few amphibians have ability to change colour by expansion and contraction of pigment cells. This phenomenon is known as Metachrosis.

Two pairs limbs help in swimming in water or moving on land. Forelimbs have four fingers and hindlimbs have five fingers.

Their digits do not have nails or claws at all.

Mouth is bigger in size. Upper or both the jaws have alike teeth. These are pleurodont, homodont and polyphyodont. Suspensorium of jaws is autostylic. (Frog - Acrodont)

A well developed and complete alimentary canal along with digestive glands are present in digestive system (Salivary glands are absent in frog).

Alimentary canal, urinary bladder and genital ducts open into cloaca.

Respiration by gills, skin, lungs or buccopharyngeal cavity.

Two nostrils are found, this condition is called dirhynous.

Heart is three chambered. 2 auricles and 1 ventricle (arteriovenous). Sinus venosus and Truncus arteriosus is well developed.

R.B.Cs are biconvex, oval and- nucleated.

In these animals, renal portal system and hepatic portal system are found.

Endoskeleton is made up of bones, but cranium is cartilaginous.

Skull has two occipital condyles, with the help of these two condyles skull is connected by first vertebra of vertebral column i.e, Atlas, this type of skull is called dicondylic skull.

Ribs absent, but may be present in some animals, but ribs are not attached with sternum.

Vertebrae, in these .animals are procoelus type, in which centrum is concave from anterior side and convex from posterior side.

External ear absent, only one ear ossicle columella (stapes) is present in middle ear.

Cranial nerves are 10-pairs.

Lateral line sensory system is necessarily found in any stage of development. In frog it is found only in larval stage.

1 pair of kidneys work as excretory organs. These kidneys are mesonephric' or opisthonephric type. These animals are ureotelic. But tailed animals & larvas are ammoniotelic.

These are cold blooded or poikilothermal animals.

These animals undergo hibernation or aestivation to prevent themselve from extreme cold and heat and to overcome unfavourable conditions.

These are unisexual animals, males have copulatory organs sometimes. These animals return to water from land for their. reproduction.

Fertilization is external and inside the water, but some animals show internal fertilization.

These are oviparous, which lay their eggs in water.

Eggs are mesolecithal. Extra-embryonic membranes are absent, so these are placed under anamniota group.

Cleavage in eggs is holoblastic and unequal.

Development is indirect type i.e. tadpole larva – In Frog: Axolotl larva - In Salamander

This class is divided into three orders:

(A)    Gymnophiona or Apoda

(B)    Caudata or Urodela

(C)    Anura or salientia

(a)    Order - Gymnophiona Of' Apoda

Their body is worm like. These are primitive amphibians burrowing in nature.

Their body is limbless.

Skin is soft, and moist, with mucous glands; small scales (cycloid) are found on the skin.

Their eyes are covered by opaque skin. (Blind)

Middle ear and tympanic membrane absent. (Deaf)

Protrusible copulatory organ is present in male.

Fertilization internal.

Parental care is found in them.

Tail ill developed or absent.

e:g. Ichthyophis (Caecilian): Blind worm without tongue.

Uraeotyphlus: Dermal scales are found in the skin.

Gymnophis

(b)    Order - Caudata or Urodela

Body is distinctly divided into head trunk and tail.Tail may have caudal fin.

Normally two pairs of limbs are found which are of equal-size. Sometimes hind limb is absent.

Skin is scale less.

Middle ear arid tympanic membrane absent.

External gills are present only in larval stage.

Copulatory organ absent in males.

Fertilization is internal.

Vertebrae are numerous their centrum is amphicoelous or opisthocoelous type.

Characterstic feature of this order is neoteny. Larva attains sexual maturity without undergoing metamorphosis, and starts reproduction.

e.g.:

Salamandra: It is viviparous. Its larva is called Axolotl larva. It sometimes show neoteny. (Longest gestation period - 36 months)

Proteus - Cave - salamander (Blind)

Ambystoma - Tiger salamander (Axolotl larva)

Triton - Newt

Neciurus - Water dog or mud puppy:- Gills in adult also. It shows permanent neoteny.

Amphiuma - Congo-eel - Largest RBC is present Siren - Mud-eel

Cryptobranches - Hell Bender: Largest Amphibian, Fully aquatic.



(c)    Order: Anura or salientia:

These are specific animals where tail is absent in adult stage.

All the frogs and toads are included in this order,

Skin is moist with mucous.

2 pairs of limbs are found. Fore 'limbs are short and hind limbs are long.

Gills are absent in aduts.

Vertebral column small, in which only 5 - 9 vertebrae are found. Last vertebra is -stick like urostyle.

Eyes with lids, tear glands present. (Lower lid movable & upper immovable).

Maxillary teeth are present in the upper jaw (absent in toad).

Middle ear and Tympanic membrane present.

Egg laying, fertilisation and development is always in water.

These have well developed vocal sacs i.e. power of voice.

Fertilization external.

Development indirect i.e. tadpole larva is found in them.

Metamorphosis complete:      

e.g.:

Bufo - Common toad [Poision glands are modification of parotid gland.         

Hyla - Tree - frog

Rana tigrina - Indian bull frog. Mentomechanical bond is found (Tip of the lower jaw).



Rhacophorus - Flying frog

Alytes - Midwife toad - Parental care is well developed in them. Male toads carry eggs in their limbs.

Pipa americana - Surinam toad - carries eggs,

Secondary vivipary. (Tongue absent)

Rana goliath - Largest frog

Phyllobates - Smallest frog (found in Cuba)

Discoglossus or Bombinator - Free bellied toad.

Xenopus - African toad



Review

Amphibia (amphibians): First land vertebrates, evolved from lobe - finned bony fishes; skin naked and moist for respiration, have four limbs, digits without claws, sac-like lungs, 3-chambered heart, undergoes metamorphosis, embryonic membranes not formed.