AQUATIC plants and animals are those that live either wholly or partly in water. The term is very vaguely used, those plants being often called A. which grow in ponds, ditches, etc., although not only their inflorescence, but great part of their foliage, is above the surface of the water, as well as those which more completely belong to that element; and a similar latitude of meaning prevails with regard to animals. Few phanerogamous (or flowering) plants exist entirely under water, although there are a few, like the common zostera. marina, or grass-wrack, which do so, and produce even their flowers in that condition; others, of which the greater part of the plant is usually under water; produce their flowers upon, or considerably above, its surface, as those of the genera valAneria, anacharis (q.v.), etc. The leaves, as well as the flowers, of many float upon the water, of which the water-lilies furnish well-known and beautiful exam ples; whilst in ranunculus aquatiliv, that exquisite ornament of our river margins, we have an instance of a kind not unfrequent, of great diversity between the lower leaves which remain submersed, and the upper leaves which float. Of crvptogamous plants, one great order, al a?, is exclusively A., and these seem adapted to perform under water all the functions of their life. A. plants are, in general, of less compact structure than is usual in other plants, and are thus lighter and better adapted for rising in their growth towards the surface of the water; in order to which also some of the algte, as may be seen in more than one of the most common sea-weeds of our coasts, arc provided with air-blad ders of considerable magnitude. All this is the more necessary, as plants completely A. have generally little firmness of stem, and if their weight made them fall to the bottom, would lie in a mass, as they do when withdrawn from the water, in which, however, they gracefully float, their flexibility of stem enabling them to adapt themselves to waves or currents, which would destroy them if they were more rigid. So admirably are all things in nature harmonized.
Many animals, to a considerable extent A. in their habits, must not only breathe air, but are adapted for spending great part of their existence on dry land. Such are chiefly
those that seek their food in the water. The peculiarities of structure by which they arc fitted for wading, for swimming, for diving, and for remaining under water a longer time than other animals can, are very interesting and admirable. Even the fur of the beaver, the otter, the water-rat, and other animals Of this description, is not liable to be drenched like that of other quadrupeds; and the plumage of water-fowls exhibits a simi lar peculiarity. The feet of many are webbed, so as to enable them to swim with great facility; and to this the general form, as in water-fowls, likewise exhibits a beautiful adaptation. The webbed feet in some, of which the habits are most thoroughly A.. as seals, assume the character of a sort of paddle, admirably fitted for use in the water, but by means of which they can only move very awkwardly on land. The forms of whales and fishes are remarkably adapted for progression in water; whilst, instead of the limbs by which other vertebrate animals are enabled to move upon the land or to fly in the air, their great organ of locomotion is the tail, or rather the hinder part of the elougated body itself, with the tail as the blade of the great oar, which all the principal muscles of the body concur to move. Remarkable provision is made in A. animals of the higher vertebrate classes for the maintenance of the requisite animal heat, by the character of the fur or plumage; a purpose which the blubber of whales also most perfectly serves. In the colder-blooded animals. where no such provision is requisite, the structure of the heart is accommodated to the diminished necessity for oxygenation of the blood; and although reptiles in their perfect state must breathe air, many of them can remain long under water without inconvenience. Fishes, and the many other animals provided with branchke or gills, breathe in the water itself, deriving the necessary oxygen, which in their case is comparatively little, from the small particles of air with which it is mingled. They cannot subsist in water which has been deprived of air by boiling. Some A. insects carry down with them into the water particles of air entangled iu hairs with which their bodies are abundantly furnished.