There is another extensive tribe of aquatic animals, which are provided with perfect articulated members, sometimes indeed supplied with fringes which convert them into a swimming apparatus, but always adapted to enable the animals to walk or crawl along the bottom. Such is the case with all the Crustaceous tribes—the crabs, lobsters, prawns, &e. ; and these animals, as is well known, can walk on dry land with the same ease as at the bottom of the ocean. When they swim, it is by means of the tail, which is always constructed for that special purpose, and is large and powerful.
Nor is the modification of structure less striking when we examine those land-animals which breathe air, and resort only occasionally to the water. As they are intermediate in habits, so are they likewise intermediate in structure between these two extremes; and the degree in which their organisation is modified, when compared with either of the two types, is exactly proportioned to the difference of their habits and economy. All Mammals and Reptiles, for instance, which seek their food in fresh-water rivers and lakes, partake more of terrestrial than of aquatic habits. The extent of water with which they are conversant is, in this case, very small when compared to the extent of land, and their organisation differs but slightly from that of ordinary laud animals ; their extremities are perfectly developed, and of the ordinary form, the principal difference being that their toes are united by an expanded web or membrane, which gives the paw a broad oar-like form, and thus converts it into a convenient instrument of swimming, at the same time that it scarcely interferes with the most perfect freedom of walking and running on land. Of this nature are the extremities of all the vertebrated terrestrial animals which seek their food in fresh water, the otters, beavers, &c. among mammals; the whole order of Natatores, comprising the ducks, swans, pelicans, gulls, auks, puffins, &c. among the birds; and the crocodiles, alligators, fresh-water tortoises, and frogs, among the reptiles. All these animals aro, properly speaking, web-footed, and their aquatic habits are less prominent and powerful than their terrestrial ; their organs of motion in fact arc but little different from those of common terrestrial animals. In those which frequent the salt-water, on the contrary, the aquatic habits greatly predominate over tho terrestrial : they live less on land than in water and the structure of their extremities approximates more to that of purely aquatic than of terrestrial anirnale. Their legs are short, and inserted,
or as it were buried, in the common integuments of the body, as far as the elbows and knees respectively, leaving apparent only a short fin-like paw, which is unadapted to terrestrial progression, exactly in proportion to its fitness as an organ of swimming. Their progress on land is consequently slow and difficult ; they creep rather than walk, dragging the body along the ground, and leaving a broad mark behind them. Few species possess even this limited power of terrestrial motion; those which do however have the structure of the extremities a little less approximated to the form of fins than the purely oceanic species. The seals and walruses, for instance, have the bones of the paws and feet similar to those of ordinary land-quadrupeds, only much shorter and more flattened, and the hind legs are thrown backwards in the same direction as the taiL Still they are enabled to use the extremities iu a certain degree for walking or creeping on dry land ; but the numerous tribes of eetaceous animals, which can execute no kind of motion whatever out of the water, have the bones of the anterior extremities flattened and connected together like the stones of a mosaic pavement, whilst the posterior members are entirely wanting. The same is the case with the sea-tortoises, or, as they are more properly called, turtles, when compared with those which frequent fresh-water ponds and rivers ; the form of their extremities approximates more nearly to that of fins than of feet, and their aquatic habits constantly predominate over their terrestrial.
Thus it is that the peculiar form of the extremities not only indicates the degree in which an animal is aquatic, but even the nature of the element which it frequents. If it inhabits fresh-water ponds and rivers, its feet are simply webbed between the toes, but in other respects perfectly developed, and its terrestrial habits predominate over its aquatic; if, on the contrary, it inhabits the salt water, its feet are flattened into the form of fins, the hind legs are thrown backwards into the plane of the body, and the aquatic habits greatly predominate over the terrestrial. The first are, properly speaking, web-footed, this second lin footed.