英文科學(xué)讀本 第四冊·Lesson 56 The Mouths of Animals

Lesson 56 The Mouths of Animals

Just as the limbs of animals are specially adapted to their mode of locomotion, so are the teeth, the form of the mouth, and even the movements of the jaw adapted to the kind of food the creatures eat. It will be very advantageous at this stage to compare the various classes of vertebrate animals in these respects.

Taking first the Insectivora—bats, hedgehogs, and moles—we find the teeth bristling with sharp points, for crushing the hard horny cases of beetles and other insects, which form their natural food. They feed on insects, not by accident, but because their teeth are formed to crush such food. Some of the bats live on fruit. Their teeth have no sharp cutting points, but are broad and rounded for grinding purposes.

Passing next to the Carnivora, we notice in all of them the enormous development of the four canine teeth, and the sharp cutting edges of the others. The great canine teeth are meant for seizing the prey; the sharp jagged edges of the other teeth are for cutting through the flesh; and this work is still further assisted by the peculiar up-and-down movement of the jaw.

In the cat and dog families the tongue is long, fleshy, and very flexible. These animals drink by lapping with the tongue. In the cats the tongue is furnished with sharp horny spikes on its upper surface, and is used for rasping the flesh from the bones. The dog's tongue is smooth. He perspires through his tongue.

The seal feeds mostly on fish. Its teeth are furnished with sharp saw-like edges, to serve the double purpose of seizing and holding such slippery prey.

In the Rodents (gnawing animals) the four incisor teeth become the chief peculiarity. These teeth are of more rapid growth than the teeth of most animals; they develop into sharp-edged chisels; their very work of gnawing tends to sharpen them for further use. The cleft in the upper lip is designed to assist in the work of gnawing. There are no canines. Between the last of the incisors and the first of the molars, in each jaw, there is a space where the canine teeth of other animals are set. The molars are meant for grinding, not cutting, and the movements of the jaw, from side to side, as well as up and down, are designed to assist in the work.

The mouths of the Ruminants (cud-chewers) present their peculiarity, and it is in the entire absence of incisors in the upper jaw. The place of these teeth is occupied by a hard gristly pad, which the animals use in collecting up and tearing off the tufts of grass, preparatory to the act of swallowing them. The molars are all largely developed, but they have broad crowns for grinding, and the jaw has the usual double movement designed for the same purpose.

The so-called Toothless animals differ considerably among themselves, according to the nature of their food. In none of the family do we find canine teeth; in some the incisors are wanting; and others, again, are entirely without teeth. In most of them the lips are hard and sufficiently horny to crush the insects and soft-bodied animals on which they live.

It is worthy of notice here that many of the insect-feeding animals—mammals, batrachia, and reptiles—have tongues specially fitted to assist in securing their prey. The tongue is not only long and flexible, but is capable of being thrust for a considerable distance out of the mouth. As a further aid in capturing their insect prey, the tongue is generally covered with a slimy, gummy fluid, which holds the victims fast while they are being conveyed to the mouth.

Among the Cetacea (whale-like animals) the great Greenland whale presents a remarkable peculiarity. Its mouth is entirely destitute of teeth; but hanging from the roof of the mouth, downwards, are a number of transverse horny plates of whalebone. These plates act the part of a trap, in which to entangle the small creatures that form the natural food of this sea monster. Birds, as we know, are without teeth. Their bills serve instead of teeth. Fishes and reptiles have usually a large number of small pointed teeth, all directed backwards, for the purpose of seizing and holding their prey, which they swallow whole.