anatomical point of view these may represent very different parts. In the Whales, the anterior extremities and the tail are transformed into fins. In Fishes, the pectoral fins, which represent the arms, and the ventral fins, which represent the legs, are employed for swimming, but they are not the principal organs; for it is by the tail, or caudal fin, that progression is principally effected. Hence the progression of the fish is precisely that of a boat under the sole guidance of the sculling-oar. In the same manner as a succession of strokes alternately right and left propels the boat straight forwards, so the fish advances by striking alternately right and left. To advance obliquely, it has only to strike a little more strongly in the direction opposite to that which he wishes to take. The Whales, on the contrary, swim by striking the water up and down; and it is the same with a few fishes also, such as the rays and the soles. The airbladder facilitates the rising and sinking of the fish, by enabling it to vary the specific weight of the body.

196. Most land animals swim with more or less ease, by simply employing the ordinary motions of walking or leaping. Those which frequent the water, like the beaver, or which feed on marine animals, as the otter and duck, have webbed feet; that is to say, the fingers are united by a membrane, which, when expanded, acts as a paddle.

197. There is also a large number of invertebrate animals in which swimming is the principal or the only mode of progression. Lobsters swim by means of their tail, and, like the Whales, strike the water up and down. Other crustacea have a pair of legs fashioned like oars; as the posterior legs in sea-crabs, for example. Many insects likewise, swim with their legs, which are abundantly fringed with hairs to give them surface; as the little water boatmen, (Gyrinus, Dytiscus,) whose mazy dances on the summer streams every ɔne must have observed. The cuttle-fish uses its long ten

tacles as oars, (Fig. 47;) and some star-fishes (Comatula, Euryale) use their arms with great adroitness, (Fig. 151.) Finally, there are some insects which have their limbs constructed for running on the surface of water, as the waterspiders, (Ranatra, Hydrometra.)

198. A large number of animals have the faculty of moving both in the air and on land, as is the case with most birds, and a great proportion of insects. Others move with equal facility, and by the same members, on land and in water, as some of the aquatic birds and most of the reptiles, which latter have even received the name Amphibia, on this account. There are some which both walk, fly, and swim, as the ducks and water-hens; but they do not excel in either mode of progression.

199. However different the movements and offices performed by the limbs may appear to us, according to the element in which they act, we see that they are none the less the effect of the same mechanism. The contraction of the same set of muscles causes the leg of the stag to bend for leaping, the wing of the bird to flap in the air, the arm of the mole to excavat? the earth, and the fin of the whale to strike the water.

CHAPTER SIXTH.

NUTRITION.

200. THE second class of the functions of animals are those which relate to the maintenance of life and the perpetuation of the species; the functions of vegetative life, (59.)

201. The increase of the volume of the body must re quire additional materials. There is also an incessant waste of particles which, having become unfit for further use, are carried out of the system. Every contraction of a muscle expands the energy of some particles, whose place must be supplied. These supplies are derived from every natural source, the animal, vegetable, and even the mineral kingdoms; and are received under every variety of solid, liquid, and gaseous form. Thus, there is a perpetual interchange of substance between the animal body and the world around. The conversion of these supplies into a suitable material, its distribution to all parts, and the appropriation of it to the growth and sustenance of the body, is called NUTRITION in the widest sense of that term.

202. In early life, during the period of growth, the amount of substances appropriated is greater than that which is lost At a later period, when growth is completed, an equilibrium between the matters received and those rejected is established. At a still later period, the equilibrium is again disturbed, more is rejected than is retained, decrepitude begins, and at last the organism becomes exhausted, the functions cease, and death ensues.

203. The solids and fluids taken into the body as food are

subjected to a process called Digestion, by which the solid portions are reduced to a fluid state also, the nutritive separated from the excrementitious, and the whole prepared to become blood, bone, muscle, &c. The residue is afterwards expelled, together with those particles of the body which require to be renewed, and those which have been derived from the blood by several processes, termed Secretions. Matters in a gaseous form are also received and expelled with the air we breathe, by a process called Respiration. The nutritive fluids are conveyed to every part of the body by currents, usually confined in vessels, and which, as they return, bring back the particles which are to be either renovated or expelled. This circuit is what is termed the Circulation. The function of Nutrition, therefore, combines several distinct processes.

SECTION I.

OF DIGESTION.

204. Digestion, or the process by which the nutritive parts of food are elaborated and pre

pared to become part of the body, is effected in certain cavities, the stomach and intestines, or alimentary canal. This canal is more or less complicated in the various classes of animals; but there is no animal, however low its organization, without it, in some form, (54.)

205. In the polypi, the digestive apparatus is limited to a single cavity. In the Sea Anemone, (Ac

Fig. 48.

tinia,) for example, it is a pouch, (Fig 48, b,) suspended in

the interior of the body. When the food has been sufficiently digested there, it passes, by imbibition, into the general cav. ity of the body, (c,) which is filled with water, and mingling

with it, flows thence into all parts of the animal. The jelly-fishes, (Medusæ,) and some Worms, have a distinct stomach, with appendages branching off in every directior, (Fig. 31,) in which a more complete elaboration takes place. The little worms known by the name of Planaria, present a striking example of these ramifications of the intestine, (Fig. 49, e.) But here, likewise, the product of digestion mingles with the fluids. of the cavity of the body which surround the intestine (d) and its branches, and circulation is not yet distinct from digestion.

Fig. 49. 206. As we rise in the scale of animals, the functions concerned in nutrition become more and more distinct from each other. Digestion and circulation, no longer confounded, are accomplished separately, in distinct cavities. The most

important organs concerned in digestion are the stomach, and the small and large intestine. The first indications of such a distinction are perceived in the higher Radiata, such as the sea-urchins, (Fig. 50,) in which the stomach (s) is broader than either extremity of the intestine. The dimensions and form of the cavities of the intestine vary considerably, according to the mode of life of the animal; but the special functions assigned to them are invariable; and the three principal cavities succeed each other, n

Fig. 50.