AQUATIC ANIMALS. The element in which animals habitually reside, or to which they occasionally resort for the purpose of pro curing food or seeking shelter, is so intimately connected with, and bears ,so obvious a relation to, not only their manners and economy, but likewise their outward forms and internal structure, that it is not surprising that those who first turned their attention to ths study of zoology, and sought to introduce the principles of classification into the animal kingdom, should have been so forcibly struck with its importance as to have made it the primary basis of their system. "Animals," says Aristotle (` Hist.' b. i. a I), "may be distributed into different classes according to their manner of living, their actions, their character, and their parts.. . . . Considered according to their manner of living, their actions, and their character, they are divided into terrestrial and aquatic. The aquatic are divided into two classes; the one, as is the case with many fishes, pass their whole life in the water, breathe that element, and find their food in it ; nor do they ever leave it : the others obtain their food in the water, and even habitually reside in it, but they do not breathe it ; they breathe air, and bring forth their young on dry land. Among these latter some are provided with feet and walk upon dry land, others have wings and fly, and others, like the water-serpent, have no feet. . . . . Aquatic animals inhabit seas, lakes, marshes, and rivers." These principles of classification, in which the habits of animals take prece dence of those modifications in their organic conformation which produce these very habits, have long since ceased to be adopted by scientific naturalists; notwithstanding which there is perhaps no inquiry which can engage the attention of the zoologist more fruitful in extensive views and interesting results than the consideration of the organic structure of animals in relation to the clement in which nature has ordained them to live.
Those animals which reside entirely in the water, and seek their food and nurture their young in that element, have their organisation, even to the most minute circumstance, rigidly adapted to these pur poses. The extremities by which progressive motion Is performed in the acts of walking and flying would be a serious impediment to the movements of animals residing in an element of the same specific gravity as their own bodies : these organs accordingly are either entirely wanting, or are reduced to mere rudiments, which serve indeed to keep the body steady and preserve its equilibrium,-but are entirely useless in assisting its progression. Such are the fins of fishes, and the flippers, as they are called, of the Whale. The real organ of progression in both cases is the body itself, which is prolonged and attenuated towards the tail, compressed on the sides, and provided with extremely powerful muscles, with which, by alternately striking the water on either side, the animal propels itself forward with a force and velocity unexampled in any other class of animated beings. It is
upon this principle that a boat is urged forWard by means of a single oar in the stern. The great majority of these animals not only reside habitually in the water, and seek their food there, but likewise breathe that element, and are consequently furnished with an appropriate apparatus for extracting the oxygen gas from its general mass. These tribes may reside at any depth of the ocean and for any length of time ; they are not under the necessity of coming frequently to the surface for the purpose of breathing, and their organisation is modified accordingly. Instead of having the tail broad horizontally, it is broad in a vertical direction, which enables them to turn with astonishing rapidity, and is no impediment, but rather an assistant to their forward movements. But the ease is different in the Whales and allied animals, which, though residing entirely in the water, breathe air by means of lungs like ordinary Mammalia, and are consequently obliged to come continually to the surface. For this purpose they are provided with a powerful cartilaginous tail flattened horizontally, by moving which upwards or downwards as the occasion requires, they ascend to or descend from the greatest depths of the ocean with almost incredible speed. Fishes, though capable of proceeding straight forwards, or of turning with great rapidity, arc comparatively slow in changing their depths; and if they breathed air, they would frequently be suffocated before they could arrive at the surface, from the vertical position of the tail not being adapted to propel them in a vertical direction. But by a simple change, merely by the direction of the tail being altered from the vertical to the horizontal position, the object of nature is accomplished, and the air breathing Cotaceous Animals are adapted to all the circumstances of an aquatic life. Another beautiful adaptation is observed in the position of the Maralilie, for the Ceiacea, liko warm-blooded quadru peds, suckle their young ; these are situated upon the breast, and when the young animal requires to suck, the mother stands, as it were, upright in the water, with her head and shoulders elevated above the surface, supporting herself by means of her flippers, or fere paws. In this position she is enabled to supply her cub with the food which nature has provided, and which she could not have accomplished had the mammw been placed in any other position.