General
Characteristics and Classification of Amphibia up to order
Amphibians are the first vertebrates adapted for both terrestrial and aquatic life. However, they are not completely adapted to life on land and exist in a state of transition between aquatic and terrestrial environments. The name "Amphibia" reflects this dual lifestyle (from Greek: "amphi" meaning dual or double, and "bios" meaning life). Structurally, amphibians are intermediate between fish and reptiles.
General Characteristics:
- Habitat: Aquatic or semi-aquatic (mostly freshwater); capable of breathing air and water; carnivorous; cold-blooded (poikilothermic); oviparous; tetrapod vertebrates.
- Body Structure: Distinct head and elongated trunk. Neck and tail may be present or absent.
- Limbs: Typically two pairs of limbs (tetrapod); some species are limbless. Toes number 4-5 (pentadactyle) or fewer. Paired fins are absent. Median fins, if present, lack fin rays.
- Skin: Soft, moist, and glandular with pigment cells (chromatophores).
- Exoskeleton: Absent. Digits are clawless. Some species possess concealed dermal scales.
- Endoskeleton: Mostly bony. The notochord does not persist. The skull has two occipital condyles.
- Mouth and Teeth: Large mouth with small homodont teeth on the upper or both jaws. The tongue is often protrusible. The alimentary canal ends in a cloaca.
- Respiration: Through lungs, skin, and the lining of the mouth. Larvae have external gills, which may persist in some aquatic adults.
- Circulatory System: Three-chambered heart (two atria and one ventricle). Sinus venosus present. Aortic arches range from 1 to 3 pairs. Well-developed renal and hepatic portal systems. Large, oval, nucleated erythrocytes.
- Excretory System: Mesonephric kidneys. Large urinary bladder. Urinary ducts open into the cloaca. Excretion is ureotelic.
- Nervous System: Brain is poorly developed with 10 pairs of cranial nerves.
- Respiratory Openings: Nostrils connected to the buccal cavity.
- Auditory System: Middle ear with a single rod-like ossicle, the columella.
- Sensory System: Larvae and some aquatic adults have a lateral line system.
- Reproduction: Sexes are separate. Males lack a copulatory organ. Gonoducts open into the cloaca.
- Fertilization: Mostly external. Females are generally oviparous.
- Development: Indirect development with holoblastic but unequal cleavage. No extraembryonic membranes. The larva, typically a tadpole, undergoes metamorphosis into an adult.
Classification of Amphibia
There are about 2,500 living species of amphibians. Amphibians were dominant during the Carboniferous period, but many species have since gone extinct. In 1924, G. Kingsley Noble recognized three orders of extinct amphibians and three orders of living amphibians.
Subclass I: Stegocephalia (Extinct)
Characteristics:
- Limbs pentadactyle (five-toed).
- Skin covered with scales and bony plates.
- Skull with a solid bony roof, leaving openings only for eyes and nostrils.
- Existed from the Permian to the Triassic periods.
Order 1: Labyrinthodontia
Characteristics:
- Considered the oldest known tetrapods, termed stem Amphibia.
- Inhabited freshwater or land.
- Resembled salamanders or crocodiles.
- Teeth were large with highly folded dentine, similar to their crossopterygian ancestors.
- Lived from the Carboniferous to the Triassic periods.
Example: Eryops.
Order 2: Phyllospodyli
Characteristics:
- Small, salamander-like in appearance.
- Large, flat heads.
- Tubular vertebrae.
- Notochord and spinal cord housed in a common cavity.
- Believed to be ancestors of modern Anura (frogs) and Caudata (salamanders).
- Existed from the Carboniferous to the Permian periods.
Example: Branchiosaurs (Ichthyostega).
Order 3: Lepospondyli
Characteristics:
- Small, salamander-like or eel-like.
- Cylindrical vertebrae, each made of a single piece.
- Neural arch and centrum are continuous.
- Ribs articulate intervertebrally.
- Considered ancestral to modern caecilians (Gymnophiona).
- Existed from the Carboniferous to the Permian period
Subclass II: Lissamphibia (Living)
Characteristics:
- Modern amphibians lacking a dermal bony skeleton.
- Teeth are small and simple.
Order 1: Gymnophiona (or Apoda)
(Greek: gymnos = naked + ophioneos = serpent-like or Greek: a = without + poda = foot)
Characteristics:
- Limbless, blind, elongated, worm-like, burrowing tropical forms known as caecilians.
- Tail is short or absent; cloaca is terminal.
- Some have dermal scales embedded in their transversely wrinkled skin.
- Skull is compact and roofed with bone.
- Limb girdle is absent.
- Males have protrusible copulatory organs.
Examples: About 55 species, including Ichthyophis and Uraeotyphlus.
Order 2: Urodela (or Caudata)
(Greek: ura = tail + delos = visible or Latin: cauda = tail)
Characteristics:
- Lizard-like amphibians with a distinct tail.
- Usually have two pairs of limbs, which are typically weak and almost equal in size.
- Skin lacks scales and a tympanum.
- Gills may be permanent or lost in adults.
- Males lack copulatory organs.
- Larvae are aquatic and resemble adults, having teeth.
Examples: Salamandra (Salamander), Desmognathus, Amphiuma etc.
Order 3: Salientia (or Anura)
(Latin: saliens = leaping or Greek: an = without + aura = tail)
- Specialized amphibians without tails in adults.
- Hind limbs are usually adapted for leaping and swimming.
- Adults lack gills or gill openings.
- Well-formed eyelids; tympanum present.
- Skin is loosely fitting and scaleless.
- Mandible is toothless.
- Pectoral girdle is bony; ribs are absent or reduced.
- Vertebral column is very short, consisting of 5-9 presacral vertebrae and a slender urostyle.
- Fertilization is always external.
- Fully metamorphosed without neotenic forms.
- About 2,200 species of frogs and toads distributed in 5 suborders.
Example: Bombinator, Discogiosus, Pipa, Xenopus, Bufo (common toad), Rhinoderma, Dendrobates, Hyla (tree toad), Alytes etc.
