Amphibians are those groups of animals which attempts the transitional mode of life between aquatic and terrestrial mode of life. From phylogenetic history this class is placed in between the Fishes and Reptiles.
General Features of the Class:
(i) Aquatic, semi-aquatic, carnivorous, cold blooded animals.
(ii) Body is distinguished into head and trunk, neck and tail may or may not present.
(iii) Presence of four pentadactyl limbs finger or webs may present, fins without rays.
(iv) Skin soft, moist, pigmented, and glandular.
(v) Edoskeleton bony skull with 2 occipital condyl.
(vi) Notochord is embryonic does not persist.
(vii) Mouth large, contains protusible tongue and teeth homodont.
(viii) Digestive system complete.
(ix) Respiration by lungs and skin, sometimes by gills (in larva).
(x) Heart three chambered, sinus vonosus present, portal systems well developed.
(xi) Kidney mesonephiric, excretion ureotelic.
(xii) Brain poorly developed cranial nerves 10 pairs.
(xiii) Sexes separate, without copulatory organ fertilization external, oviparous.
(xiv) Development indirect, cleavage homoplastic but unequal.
(xv) Larva is tadpole larva transform into adult through metamorphosis..
There are about 2,500 species under six orders, out of which 3-order extinct and 3-order is living.
Classification of Amphibia :
According to G.Kingsley Nobel, class Amphibia is sub-divided into two sub-classes one includes all extinct order and other sub-class includes the living members.
(A) SUB-CLASS-I: STEGOCEPHALIA (Extinct).
i) Limbs are pentadactyle. (five digitate)
ii) Skin covered by scales and bony plates.
iii) Skull with a solid bony roof which posses two pairs of opening for eyes and nostril.
iv) They were pound between Permian to Triassic period of geological time scale.
This sub-class is further grouped into three orders
(I) Order-I – Labyrithodontia:
(i) These groups of amphibians are the most primitive tetrapods and oldest known tetrapods.
(ii) These are called as stem Amphibia.
(iii) Both water and land formed.
(iv) Shows characteristic of Crocodiles and Salamander.
(v) Skull completely roofed over by bones.
(vi) Much bony element present in their skulls than the modern amphibia.
(vii) Enlarged teeth with greatly folded definite found in both the jaw.
(viii) Definite similar to their crossopterygian ancestors.
(ix) They lived from Lower Carboniferous era to the Triassic periods.
Example: – Eryops, Ecogyrinus, Palaeogyrinus, Cyclotosaurus.
(II) ORDER – II – PHYLLOSPODYLI:
(i) Small salamander like body.
(ii) Head large and flat.
(iii) Vertebrae tubular.
(iv) Notochord and spinal cord housed in same cavity.
(v) Well marked transverse process and stout ribs.
(vi) Pubis cartilaginous.
(vii) Four finger in limbs with five toes.
(viii) Coracoids cartilaginous.
(ix) Skull roof posses separate Quadratojugal and Lacrimal process.
(x) Lived during carboniferous to Permian period.
(xi) Considered as the ancestors of modern Salientia and Urodela.
Examples:– Branchiosaurus (Ichthyostega)
(III) ORDER – III – LEPOSPONDYLI:
(i) Small samander like or el like.
(ii) Vertebrae cylindrical, each made up of single piece.
(iii) Neural arch and centrum continuous.
(iv) Ribs articulating intervertebrally.
(v) Regarded as the ancestors to modern Gymnophiona.
(vi) Lived between Carboniferous to Permian period.
Example: – Diplocaulus, Lysorophus.
(B) SUB-CLASS – II – LISSAMPHIBIA:
i) This sub-class includes all modern amphibia, living forms.
ii) Dermal bony skeleton over skin absent.
iii) Teeth small and simple.
This sub-class includes three orders of living amphibia.
(I) ORDER-I: Gymnophonia or Apoda :
(i) These group of amphibian are blind, limbless, burrowing in nature.
(ii) Elongated worm like body.
(iii) Tail short or absent, cloaca terminal.
(iv) In some cases dermal scales embedded in skin which is transversely wrinkled.
(v) Skull compact roofed by bone.
(vi) Girdles absent in limbs.
(vii) Body is grooved transversely.
(viii) Eyes without eye lids.
(ix) Presence of protusible tentacles between eyes and nostril.
(x) Males posses’ protusible copulatory organ.
(xi) Fertilization internal in Typhlonectess but usually external.
(xii) There are about 55 species.
Example: Ichthyophis, Typhlonectess, Ureacotyphlus etc.
(II) ORDER – II – Urodela or caudate:
(i) Lizard like amphibians with distinct tail.
(ii) 2 pairs of equal limbs.
(iii) Skin without scales and tympanum.
(iv) Males without copulatory organ.
(v) Larva aquatic.
(vi) Teeth present in both jaws.
(vii) Usually oviparous.
(viii) Body with distinct head, trunk & tail.
Example: Ambystoma, triton, Salamandra.
(III) ORDER-III –SALIENTA Or ANURA:
(i) Amphibians those lack tails in adult.
(ii) Hindlimbs larger, stout, highly muscular adopted for leaping, jumping and swimming.
(iii) Adults without gills, but present in larva.
(iv) Eye lids well developed.
(v) Ribs absent, reduced to pectoral gridle.
(vi) Tympanum present.
(vii) Skin is loosely fitted to body, wall skin without scales.
(viii) Fertilization usually external.
(ix) Fully metamorphosed without neotonic forms.
Example: Rana, Bufo, Pipa , Alytes, Xenopus, Hyla, etc.
Amphibians are lowest and earliest tetrapods not fully adapted to terrestrial life, hence treated as the transitional group between aquatic and terrestrial mode of life.
They exhibit the characteristics with crossopterygian ancestry and traced back to fresh water carnivorous of Deonian period.