Amphibia: Overview | Zoology Optional Notes for UPSC PDF Download

Characteristics and Classification of Class Amphibia

General Characters of Amphibia

1. Adaptation to Dual Environments:

   • Amphibians are the first vertebrates adept at both terrestrial and aquatic life.
   • They hover between aquatic and land environments, reflecting their double life (amphi = dual + bios = life).
2. Structural Position:
   • Positioned between fish and reptiles structurally.
   • Aquatic or semiaquatic, air and water-breathing, carnivorous, cold-blooded, oviparous, tetrapod vertebrates.
3. Distinctive Physical Features:
   • Head distinct, elongated trunk, variable presence of neck and tail.
   • Usually two pairs of limbs (tetrapod), with 4-5 toes (pentadactyle) or less.
4. Skin Characteristics:
   • Soft, moist, and glandular skin.
   • Absence of exoskeleton, clawless digits, and some may have concealed dermal scales.
5. Skeletal Makeup:
   • Mostly bony endoskeleton; no persistent notochord.
   • Skull with two occipital condyles.
6. Respiratory System:
   • Respiration by lungs, skin, and mouth lining.
   • Larvae with external gills, possibly persisting in some aquatic adults.
7. Cardiovascular Features:
   • Three-chambered heart, sinus venosus present.
   • Well-developed renal and hepatic portal systems.
8. Reproductive Characteristics:
   • Sexes separate, males lack copulatory organs.
   • Fertilization mostly external, females mostly oviparous.
9. Developmental Process:
   • Indirect development with holoblastic but unequal cleavage.
   • Larval form, tadpole, undergoes metamorphosis into an adult.

Classification of Amphibia

1. Order 1: Labyrinthodontia

   • Oldest known tetrapod, stem Amphibia.
   • Examples: Eryops.
2. Order 2: Phyllospodyli
   • Small salamander-like forms, believed to be ancestors of modern salientia and urodela.
   • Examples: Branchiosaurs (Ichthyostega).
3. Order 3: Lepospondyli
   • Small salamander or eel-like forms, regarded as ancestral to modern caecilians.
   • Examples: Diplocaulus, Lysorophus.

Subclass II: Lissamphibia (Living)

1. Order 1: Gymnophiona or Apoda (Caecilians)

   • Limbless, blind, worm-like, burrowing tropical forms.
   • Example: Ichthyophis, Uroaeotyphlus.
2. Order 2: Urodela or Caudata (Salamanders)

   • Lizard-like amphibians with a distinct tail.

   • About 300 species in 5 suborders.

   • Suborder 1: Cryptobranchoidea
     • Most primitive, permanently aquatic.
     • Examples: Cryptobranchus, Megalobranchus.
   • Suborder 2: Ambystomatoidea
     • Terrestrial adults with two eyelids.
     • Example: Ambystoma.
   • Suborder 3: Salamandroidea
     • Vertebrae opisthocoelous, teeth on palate.
     • Examples: Triton, Salamandra, Amphiuma.
   • Suborder 4: Proteidae
     • Aquatic bottom dwellers, permanent larval forms.
     • Examples: Proteus, Necturus.
   • Suborder 5: Meantes
     • Aquatic, representing permanent larvae.
     • Examples: Siren, Pseudobranchus.
3. Order 3: Salientia or Anura (Frogs and Toads)

   • Specialized amphibians without tails in adults.

   • About 2200 species in 5 suborders.

   • Suborder 1: Amphicoela
     • Vertebrae amphicoelous, presacral 9.
     • Example: Leopelma, Ascaphus.
   • Suborder 2: Opisthocoela
     • Vertebrae opisthocoelous, scapula small.
     • Examples: Alytes, Bombinator, Discogiosus, Pipa, Xenopus.
   • Suborder 3: Anomocoela
     • Vertebrae procoelous or amphicoelous.
     • Example: Pleobates, Scaphiopus.
   • Suborder 4: Procoela
     • Vertebrae procoelous, urostyle with two condyles.
     • Examples: Bufo, Rhinoderma, Dendrobates, Hyla, Gastrotheca.
   • Suborder 5: Diplasiocoela
     • First 7 vertebrae procoelous, pectoral girdle usually fused to sternum.
     • Example: Rana, Polypedates, Rhacophorus.