food, some using them merely for seizing and securing their prey, as the lizards, frogs, crocodiles, and the great majority of fishes. In such animals, the teeth are nearly all alike in form and structure, as for instance, in the alligator, (Fig. 71,), the porpoises, and many fishes. A few of the latter, some of Fig. 71. Fig. 72. the Rays, for example, have a sort of bony pavement, (Fig. 72,) composed of a peculiar kind of teeth, with which they crush the shells of the mollusks and crabs on which they feed. 218. The Mammals, however, are almost the only verte brates which can properly be said to masticate their food. Their teeth are well developed, and pre sent great diversity in form, arrangement a Fig. 73. their mode of life; nar ely, the cutting teeth, incisors; the tusks or carnivorous teeth, canines; and the grinders, molars (Fig. 73., The incisors (a) occupy the front of the mouth, the upper ones being set in the intermaxillary bones; they are the most simple and the least varied, have generally a thin cutting summit, and are employed almost exclusively for seizing food, except in the elephant, in which they assume the form of large tusks. The canines (b) are conical, moze elongated than the others, more or less curved, and only two in each jaw. They have but a single root, like the incisors, and in the carnivora become very formidable weapons. In the herbivora, they are wanting, or when existing they are usually so enlarged and modified as also to become powerful organs of offence and defence, although useless for mastication; as in the baby roussa, &c. The molars (c) are the most important for indicating the habits and internal structure of the animal; they are, at the same time, most varied in shape. Among them we find every transition, from those of a sharp and pointed form, as in the cat tribe, to those with broad and level summits, as in the ruminants and rodents. Still, when most diversified in the same animal, they have one character in common, their roots being never simple, but double or triple, a peculiarity which not only fixes them more firmly, but prevents them from being driven into the jaw in the efforts of mastication. 219. The harmony of organs already spoken of (22-24) is illustrated, in a most striking manner, by the study of the teeth of the mammals, and especially of their molar teeth. So constantly do they correspond with the structure of the other parts of the body, that a single molar is sufficient not only to indicate the mode of life of the animal to which i belongs and show whether it feeds on flesh or vegetables, or bot, but also to determine the particular group to which it is related. Thus, those beasts of prey which feed on insects, and which on that account have been called Insectivora, such as the moles and bats, have the molars terminated by several sharp, conical points, (Fig. 74,) so arranged that the elevations of one tooth fit exactly into the depressions of the tooth opposite to it. In the true Carnivora, (Fig. 75,) on the contrary, the molars are compressed laterally, so as to have sharp, cutting edges, as in the bats; and they shut by the side of each other, like the blades of scissors, thereby di viding the food with great facility. 220. The same adaptation is observed in the teeth of her bivorous animals. Those which chew the cud, (ruminants,) many of the thick-skinned animals, (pachydermata,) like the elephant, and some of the gnawers, (rodentia,) like the hare, (Fig. 76,) have the summits of the molars flat, like mill-stones, with more or less prominent ridges, for grinding the grass and leaves on which they subsist. Finally, the omnivora, those which feed on both flesh and fruit, like man and the monkeys, have the molars terminating in several rounded tubercles, being thus adapted to the mixed nature of their food. 221. Again, the mode in which the molars are combined with the canines and incisors furnishes excellent means of characterizing families and genera. Even the internal structure of the teeth is so peculiar in each group of animals, and yet subject to such invariable rules, that it is possible to determine with precision the general structure of an animal merely by investigating the fragment of a tooth under a mi croscope. 222. Arther process, subsidiary to digestion, is called insalivation. Animals which masticate their food have glands, in the neighborhood of the mouth, which secrete a fluid called saliva. This fluid mingles with the food as it is chewed, and prepares it also to be more readily swallowed. The salivary glands are generally wanting, or rudimentary, or otherwise modified, in animals which swallow their food without mastication. After it has been masticated and mingled with saliva, it is moved backwards by the tongue, and passes down through the œsophagus, into the stomach. This act is called deglutition or swallowing. 223. The wisdom and skill of the Creator is strikingly illustrated in the means he has afforded to every creature for securing the means for subsistence. Some animals have no ability to move from place to place, but are fixed to the soil; as the oyster, the polyp, &c. These are dependent for subsistence upon such food as may stray or float near, and they have the means of securing it when it comes within their reach. The oyster closes its shell, and thus entraps its prey; the polyp has flexible arms, (Fig. 77,) capable of great extension, which it throws instantly around any minute animal that comes in contact with it. The cuttle-fish, also, has elongated arms about the mouth, furnished with ranges of suckers, by which it secures its prey, (Fig. 47.) 224. Some are provided with instruments Fig. 77 for extracting food from places which would be otherwise inaccessible. Some of the mollusks, with their rasp-like tongue, (Fig. 58,) perforate the shells of other ani mals, and thus reach and extract the inhabitant. Insects have vario is piercers, suckers, or a protractile tongue for the same purpose, (Figs. 61-64.) Many Annelides, the leeches for example, have a sucker, which enables them to produce a vacuum, and thereby draw out blood from the perforations they make in other animals. Many microscopic animals are provided with hairs or cilia around the mouth, (Fig. 65,) which by their incessant motion produce currents that bring within reach the still more minute creatures or particles cu which they feed. 225. Among the Vertebrata, the herbivora generally employ their lips or their tongue, or both together, for seizing the grass or leaves they feed upon. The carnivora use their jaws, teeth, and especially their claws, which are long, sharp even movable, and admirably adapted for the purpose. The woodpeckers have long, bony tongues, barbed at the tip, with which they draw out insects from deep holes and crevices in the bark of trees. Some reptiles also use their tongue to take their prey. Thus, the chameleon obtains flies at a distance of three or four inches, by darting out his tongue, the enlarged end of which is covered with a glutinous substance to which they adhere. The elephant, whose tusks and short neck prevent him from bringing his mouth to the ground, has the nose prolonged into a trunk, which he uses with great dexterity for bringing food and drink to his mouth. Doubtless the mastodon, once so abundant in this country, was furnished with a similar organ. Man and the monkeys employ the hand exclusively, for prehension. 226. Some animals drink by suction, like the ox, others by lapping, like the dog. Birds simply fil. the beak with water, then, raising the head, allow it to run down into the crop. It is difficult to say how far aquatic animals require water with their food; it seems, however, impossible that they should swallow their prey without introducing at the same time some water into their stomach. Of many among the lowest animals, such as the Polyps it is well |