In the worms, properly so called, there are sometimes hard parts, forming a kind of jaws or teeth ; thus in the nereis, the mouth possesses several'calcareous pieces. The ophrodite (sea-mouse) has a probos cis, furnished with four teeth, which it can extend and retract at pleasure. With in the mouth of the leech are three semi circular projecting bodies, with a sharp denticulated edge : by this apparatus the animal inflicts its wound of the well known peculiar form in the skin.
The teeth of the echinus (sea-hedge hog) are of a very singular arrangement ; a round opening is left in the shell for the entrance of the food ; a bony structure, on which five teeth are placed, fills up this aperture ; and as these parts are moved by numerous muscles, they form a very complete organ of mastication.
The stomach of the vermes is, in gene ral, a membranous bag, but in some cases its structure is more complicated. The helix stagnalis and the onchidia have gizzards. The aplysia has three strong Muscular stomachs, provided with pyra midal bony processes. The latter struc tures, together with those of the lobster and crab, present a new analogy, as Cu vier has observed, between the mem branes of the intestines and the integu ments of the body. This is particularly strengthened by the annual shedding of the lobster's teeth, when its crustaceous covering falls off.
The bulls lignaria has a very powerful stomach, containing three hard calcareous shells, by which the animal is enabled to bruise and masticate the other testacea on which it feeds.
plc THE INTESTINAL CANAL.
The intestinal canal (which is the most common part in the whole animal king dom after the stomach) is distinguished in the mammalia by two peculiarities, which depend on the mode of nutrition. It is comparatively shorter in carnivorous ani mals, and there is also in these less dif ference, to external appearance, between the small and the large intestine than in the herbivora. Yet these rules are not without their exceptioOs; for the seal has very long, and the sloth very short, intes tines ; the badger, which is not a proper carnivorous animal, and several true her bivora, as for instance, the rell-mouse, (glis esculentus,) have no distinction be tween the large and small intestine, &c.
In considering the proportionate lengths of the intestinal canal, and the relation which these bear to the kind of food on which the animal subsists, many circumstances must be taken into the ac count, besides the mere measure of the intestine. Valvular projections of the in ternal membrane ; dilatations of peculiar parts of the canal ; and a large general diameter, compensate for shortness of the intestine ; and vice versa. The structure
of the stomach must also be considered, as whether it is formed of more than one cavity ; whether the oesophagus and in testine communicate with it in such a manner as to favour a speedy transmission of the food, or whether there are ctrl de sacs which retain the aliment for a long time in the cavity. The formation of the jaws and teeth, and the more or less per fect trituration and comminution which the food experiences in the mouth, must likewise be viewed in connection with the length and structure of the alimenta ry canal.
The whole length of the canal is great er in the mammalia than in the other classes. It diminishes successively, as we trace it in birds, reptiles, and fishes, be ing shorter than the body in some of the latter animals, which is never the case in the three first classes.
In omnivorous animals, the length of the canal bolds a middle rank between those which feed on the flesh and such as take vegetable food ; thus, in the rat, its proportion to the body is as 8 to 1; in the pig 13 to 1; in a man 6 or 7 to 1. The diminution in length in the latter case is compensated by other circumstances, viz. the numerous valvula conniventes, and the preparation which the food un dergoes by the art of cookery.
In carnivorous animals, every circum stance concurs to accelerate the passage of the alimentary matter. It receives no mastication ; it is retained for a very short time in the stomach ; the intestine has no folds or valves ; it is small in di ameter : and the whole canal, when com pared to the body, is extremely short, being 3 or 5 to 1. In general there is no cacum.
The ruminating animals present the opposite structure. The food undergoes a double mastication, and passes through the various cavities of a complicated sto mach. The intestines are very long ; 27 times the length of the body in the ram. Hence the large intestines are not dilated or cellular, nor is there a czcum. The solipeda have not such a length of canal, nor is their stomach complicated ; but the large intestines are enormous, and dilated into sacculi ; and the czecum is of a vast size, equal, indeed, to the stomach. The rodentia, which live on vegetables, have a very large caccum, and a canal 12 or 16 times as long as the body. In the rat, which can take animal as well as ve getable food, the canal is shorter than in the other rodentia.