Q.Comment upon the habitats and external features of animals belonging to class, amphibia and reptilia.

Amphibia: live a dual life in both aquatic and terrestrial habitats; body has a head and trunk (tail in some); skin is moist and lacks scales; eyes have eyelids; ear shown externally as a tympanum. …

Amphibia

• The very name reflects a dual life, since amphibians can live in both aquatic and terrestrial habitats.
• The body is divided into a head and a trunk, with a tail present in some forms.
• The skin is moist and lacks scales, serving as an extra respiratory surface, and the eyes bear eyelids.
• The ear is represented externally by a membrane, the tympanum.
• Most amphibians have two pairs of limbs.

Reptilia

• Reptiles are mostly terrestrial, having moved further away from a dependence on water than amphibians.
• Their body is covered by a dry, cornified skin bearing epidermal scales or scutes — a covering that resists water loss and suits dry land.
• They lack external ear openings, the ear again being represented by the tympanum, and snakes and lizards periodically shed their scales as a skin cast. …

Method: The land-transition continuum approach

Rather than describing Amphibia and Reptilia as two separate, independent classes, place them as two POINTS along a single continuum — "degree of independence from water" — and derive each class's habitat and external features from where it sits on that continuum.

The continuum: at one end sits full dependence on water (as in fishes); at the other end sits full independence from water (as in reptiles, birds, mammals). Amphibia sits partway along this continuum, and Reptilia sits further along it, closer to the fully-terrestrial end.

Derive Amphibia's features from its midway position: since amphibians are only PARTWAY toward terrestrial independence, they should be expected to retain some water-dependent traits alongside some land-adapted ones. This predicts: a dual aquatic-terrestrial habitat (matching the very meaning of "amphibian"); skin that stays moist rather than fully sealed against water loss (consistent with only partial independence, and doubling as a respiratory surface); eyelids and an external tympanum as land-adapted sensory features gained along the way; and typically two pairs of limbs, useful on land but not yet fully specialised away from an aquatic body plan. …