AQUATIC ANIMALS, a term denoting animals living constantly in water, and also those which swim on its surface or plunge be neath it for food. While the great majority of crustaceans are aquatic, a few, such as the wood-louse and the land crab, are modified for life ashore. Among mollusks there is oc currence of both aquatic and terrestrial habit, while numerous forms illustrate the transition from the former to the latter. The ascidians are exclusively marine. Some fishes have a limited power of life out of the water, the double-breathing Dipnoi being in this connec tion especially instructive. Among many am phibians the transition from water to terra firma is seen in the individual life-history, when the fish-like gilled tadpole becomes the lunged gill-less frog; while in a few exceptional cases such as the black salamander of the Alps, the life is terrestrial from first to last, and even the young dispense with their preliminary swim as tadpoles, although a brief recapitulation of their aquatic life is still represented by a gilled stage within the body of the parent. The in stance of the gilled axolotl becoming, in the absence of sufficient water, the gill-less ambly stoma, forcibly illustrates the importance of the medium as a factor in evolution. Among reptiles there are numerous aquatic forms — chelonians, lizards, snakes and crocodiles though the absence of any gill respiration marks the progressive general adaptation to terrestrial life. While an emphatically terres trial amphibian like the tree-frog seeks a watery hole for the rearing of the young gill breathing tadpoles, the habit is reversed in such reptiles as the sea turtle, which, having re turned to the more primitive aquatic home, yet revisits the land for egg-laying purposes. The cradle of the young in both cases indicates the ancestral habit of the parent. Among the emphatically aerial birds there are cases, like that of the penguin, where the structure has become adapted to an almost exclusively aquatic life. Among mammals the sea-cow, the seal and the whale are familiar illustrations of very different types which have returned to the primeval watery home and aquatic habit, with consequent change of structure.
It is important to note the general fact that, in the water, animals are subjected to influences somewhat different in detail from those which mold their congeners ashore. Even contact with a different medium, varying in composi tion, in currents, in pressure, in contained food and oxygen, and the like, obviously involves a great diversity in structure. Modes of motion. from the swimming-bell of a medusoid con tracting and expanding in the tide, to that of the lowest vertebrates as illustrated in the pelagic tunicates, or from the paddling of worm and crustacean to that of fish and frog, duck and seal, are at once familiar adaptations to, and necessary results of, aquatic life. Sim ilarly the smooth and frequently fish-like form, of actively locomotive water animals, is a very noticeable adaptive result of the conditions of life. In the more thoroughly aquatic animals, which have remained in the primitive environment, and not merely returned to it, the blood is usually purified by being spread out on feathery gills which catch the oxygen dissolved in the water • while in ter restrial forms which have betaken themselves to an aquatic life, the ordinary direct "air breathing* is still accomplished at the surface of the water, or in some isolated cases of in sects and spiders, by means of the air entangled in their hairs, or even conveyed into their sub merged homes. The aquatic respiration of some larval insects, the power that some crus taceans and fishes have of keeping up a res piration on land with a minimum of water about their gills and, above all, the cases of the double-breathing fishes or Dipnoi, and of am phibians already referred to, are especially in structive in regard to the problem of transi tion from one medium to the other. The gen uinely aquatic animals are known to have a body temperature not much higher than that of the surrounding medium, and often survive even the freezing of the water; while in the higher warm-blooded vertebrates which have returned to an 'aquatic habit, various modifica tions, such as thick fur and plumage, water proof varnish, formation of blubber, serve as protections against the cold.
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