Aquatic Animals. Apart from any speculations as to the more or less watery nook where the first forms of life were cradled, it is worth noting that the home of almost all the simpler animals is distinctly and necessarily aquatic. While a few of the Protozoa, such as one of the Amœbæ, occur in damp places on land, or within other organisms, the vast majority live freely in the water, and the same is true of the Sponges, Cœlenterates, and Echinoderms. Among worms, however, more emphatic exceptions occur, such as the earthworm, where the structure and habit of the animal has become distinctly adapted to terrestrial life. While the great majority of crustaceans again are aquatic, a few, such as the wood-louse and the land-crab, are modified for life ashore. The crowd of insects, spiders, and myriapods are of course terrestrial or aerial, though here also the habits of some adult forms, and the life of some of the young, are distinctly aquatic. Among molluscs also there is an equally familiar occurrence 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 (q.v.) being in this connection especially instructive. Among many amphibians, 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 instance of the gilled Axolotl (q.v.) becoming, in the absence of sufficient water, the gill-less Amblystoma, forcibly illustrates the importance of the medium as a factor in evolution. Among reptiles there are numerous aquatic forms—cheloniæ, lizards, snakes, and crocodiles, though the absence of any gill-respiration marks the progressive general adaptation to terrestrial life. While an emphatically terrestrial 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 returned 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. And so 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.
To sum up the adaptations to aquatic life would obviously be to attempt to compress a large department of comparative physiology. It is more important simply to note the general fact that, in the water, animals are subjected to influences somewhat different in detail from those which mould their congeners ashore. Even contact with a different medium, varying in composition, 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 contracting 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. Similarly, the smooth and frequently fish-like form, especially 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 have 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 terrestrial 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 insects and spiders, by means of the air entangled in their hairs, or even conveyed into their submerged homes. The aquatic respiration of some larval insects, the power that some crustaceans and fishes have of keeping up a respiration on land with a minimum of water about their gills, and above all, the cases of the double-breathing fishes or dipnoi, and of amphibians already referred to, are specially instructive in regard to the problem of transition from one medium to the other. The genuinely 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 modifications, such as thick fur and plumage, waterproof varnish, formation of blubber, serve as protections against the cold. The sensitiveness of many forms to changes in the volume, movement, and composition of the water, the importance of the aquatic habit in relation to the dispersion of types, the power exhibited by some of the lower animals in avoiding death or at least extinction during drought, will be discussed in the articles on ENVIRONMENT, DISTRIBUTION, and DESICCATION.