Aquatic Animals
animals living in or about water. 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, cælenterates, and echinoderms .
Although the great majority of crustaceans are aquatic, a few, such as the wood-louse and land crab, are modified for life ashore. The crowd of insects, spiders, and myriapods are of course terrestrial or aërial, though here also the habits of some adult forms, and the life of some of the young are distinctly aquatic. Among mollusks also there is an equally familiar occurrence of both aquatic and terrestrial habit, while nu- AQUATIC PLANTS Numerous AQUEDUCT merous forms illustrate the transition the surrounding medium. from the former to the latter. The ascidians are exclusively marine. fishes have a limited power of life out of Some the water, the double-breathing dipnoi 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 fishlike gilled tadpole becomes the lunged gill-less frog. The instance of the gilled axolotl 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-chelonians, 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. 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. plants are, moreover, in the strict sense quired or having lost all direct connecof the word aquatic, having never action with the soil. The simplest plants or algæ are almost all aquatic, though many occur in damp situations on land, or on other organisms, while others remain for long periods quiescent in comparative dryness. Many algæ are absolutely isolated in the water, while others are more or less intimately fixed to some solid substratum. such as salvinia, are aquatic, with leaves Some rhizocarps, rising to the surface, while others are land or marsh plants, like the higher horse-tails and club-mosses.
In the more thoroughly aquatic animals, which have remained in the prim- , 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 coninto their submerged homes. 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 the higher warmblooded vertebrates which have returned to the aquatic habit, various modifications, such as thick fur and plumage, waterproof varnish, formation of blubber, serve as protections against the cold.