Following adaptations are seen in secondary aquatic adaptations

Secondary aquatic forms are lung breathers which under stress of circumstances were forced to return to water and readapted themselves to aquatic habitat. They, however, retained higher brain and efficient methods of aeration of blood.

Class-Amphibia includes the amphibious forms that spend part of their time on land and part is the water. They show only partial adaptations in webbed feet, laterally compressed swimming tail and gills. In reptiles marine turtle, crocodiles and alligators show semiaquatic adaptations with respiratory aids.

But for the egg-laying female has to come to land. Ichthysauria (extinct reptile) were fully aquatic. Among birds quite a few forms are amphibious such as Pelican, flamingo, bucks, geese etc. The Penguins are aquatic. Among mammals some forms are amphibious like sea-otter, otter, mink, hippopotamous etc. and others like cetacean and sirenian are aquatic.

Following adaptations are seen in secondary aquatic adaptations:

1. Body contour. It is also stream-lined; neck shortens or disappears, tail enlarges, a fish-like form is developed. Several modifications viz., skull modification, neck shortening vertebrae are biconcave as in fishes, sacrum is more of less reduced, bones become light and spongy, loss of external ears, external nostrils or nares move towards the apex of head, eyes shift higher no face etc., (as in hippopotamous).

2. Locomotive mechanism. Fleshy fin like expansions of the body wall without skeletal support occur in the whales and Ichthyosaurs (extinct). These fins may be dorsal and caudal.

Two types of propulsion are seen. In oar propulsion the limbs are nearly equivalent in size, e.g. in turtles and Plesiosaurs (extinct). In tail propulsion the hind limbs tend to disappear externally, e.g., in whales and sirenians. It is characteristic of secondary vertebrates that the unpaired fins are never supported by the skeletal elements or fin rays.

3. Limbs. Webbed feet is another aquatic adaptation in aquatic forms, they develop into paddles in which there is a loss of mobility of the various joints so that a flexible paddle of great aquatic utility is formed. As a further modification, the individual phalangeal bones increase in number.

4. Integument. Skin is naked i.e. there is the reduction of armouring of hair, skin glands, muscles and nerves. The loss of hair is characteristic of marine mammals. The whales and sirenians have lost all the traces of hair except of few bristles around the mouth, but most of them are well covered with partial hair before birth.

As a compensation of the loss of hairs a layer of fat or subcutaneous tissue develops beneath the skin called blubber for the retention of bodily heat.

5. Mouth armament. Except in sea-cows and walrus the jaws are not used for mastication but only for the prehension of feeble prey. Teeth become simplified, increase in number or total loss from one jaw (sperm whale) or both (baleen whale). The reptiles have simple prehensile teeth fitted for the retention of slippery prey.

6. Mental precocity. It is a necessity for the aquatic animals and they soon show ability to keep up with the mother. The new born young of whales, much smaller to their parents, is capable to maintain its position.

7. Speed. Some aquatic animals have a very fast speed in water. This speed is maintained with the vibrations of highly efficient propelling tail.

8. Size. Water-borne animals are larger in size. The largest terrestrial animals are elephant measures 3.5 meter in length while the aquatic sulphur-bottom whale (Balaenoptera musculus) is about 26.10 meter is length several times bigger than the largest elephant.