Popular Science Monthly/Volume 17/June 1880/How Animals Eat
|HOW ANIMALS EAT.|
IN the digestion of the higher animals, the first act is the trituration of the food to assist and hasten its solution. We might term this mechanical digestion, as it is a reduction of the food preparatory to the essential physical and chemical process. Although very simple in
Fig. 1.—Organs of the Mouth in Insects: 1. Trophi of a masticating insect (beetle): a, labrum or upper lip; b, mandibles; c, maxillæ with their palpi; d, labium or lower lip with its palpi.
2. Mouth of a butterfly: o, eye; f, base of antennæ; g, labial palp; h, spiral trunk or "antlia."
3. Mouth of a hemipterous insect (Nepa cinerea): l, labium; m, maxillæ; n, mandibles.
purpose, the manner is exceedingly various and interesting. The mastication of food is usually accomplished by a mill in the mouth. And absence of a masticatory apparatus at the entrance of the food-canal is usually compensated by a mill within the body, as the gizzard of a bird, or by strong chemical solvents as shown in the snake.
To soften the food and so assist in its trituration, nearly all animals are provided with saliva. But, as this fluid serves to lubricate the food for swallowing, and also has a chemical power, it may be considered in connection with digestion.
Those animals which subsist wholly on liquids or on minute particles require, of course, no masticating organs. Of the first class, are most parasites, the butterflies and some hummingbirds; and, of the second, bivalve mollusks and the whalebone whale. The little sea- chin is remarkable as the first animal, and below mollusks and articulates the only animal, possessing any organs for mastication. Furthermore, its dental apparatus is perhaps of greatest complexity in the whole animal kingdom. Five triangular teeth set in as many jaws surround the opening to the stomach, and move toward a central point. They seize and divide the food which then passes between the masticating jaws. The latter are grooved, and, aided by saliva, effectually crush and grind the food. The muscles and levers necessary to efficient action are numerous and intricate, altogether forming an apparatus entirely unique in form and principle.
Among mollusks the gasteropods or snails, and the cephalopods, which include the cuttle and devil-fishes, have masticating organs. Most snails have thousands of minute teeth on the tongue. These are, however, chiefly used for procuring food by a rasping or boring motion. But some snails, not satisfied with lingual teeth alone, indulge the absurdity of carrying powerful teeth in the stomach. The beak of the cuttle-fish divides the food somewhat, but the masticating process is mainly performed by a gizzard.
Articulates have a much greater variety of masticating organs. Nearly all, even some of the worms, have efficient jaws for seizing food, which also serve to cut and crush the same.
Imagine a creature with so little regard for the proprieties as to chew food with its legs! Yet the common limulus, or "horseshoe crab," actually grinds the food between its thighs. However, this is not so strange, when we understand that the claws of the lobster and crab are simply modified legs; and that many higher animals, the squirrel and cat, for instance, use their feet in eating.
Another group of crustaceans, the little rotifers, pound their food. The lower jaw serves the purpose of an anvil upon which the food is crushed by two hammers formed from the upper jaw. Lobsters and crabs masticate with their horny jaws; but they all have complete and efficient sets of teeth in the gizzard-like stomach. And, when the shell is cast, the teeth are also shed along with the stomach-lining. Those members of the spider and centiped classes which eat solid matter have masticating jaws. But the best development of such jaws is found in the insects. Nearly all insects while in the larval state, and many adult insects, as beetles and grasshoppers, have two pairs of
|Fig. 4.||Fig. 5.|
|Lower Jaws of Ancient Toothed Birds (after Marsh).|
|Fig. 4: Ichthyornis dispar. Twice the natural size. Teeth in distinct sockets. Fig. 5: Hesperornis regalis. One half natural size. Teeth set in a groove.|
jaws, called mandibles and maxillae, which move horizontally between upper and under lips. Such insects usually have gizzards to complete the mechanical process. The grasshoppers and others have the gizzard armed with rows of horny teeth. The activity and efficiency of insect-jaws is very great, as many people have the misfortune to know. From before the time of Pharaoh, the biting insects have been a scourge to the farmer. Their voracity is awful; and, when urged by hunger, few substances can withstand their jaws. In countless hosts they ravage a country, and blast it as with the curse of Jehovah. It is stated that some caterpillars eat three to four times their weight of food every day. That beasts like the elephant and tiger are not voracious in proportion to their size is matter for congratulation. Fortunately, however, such ravenous insects eat only in the larval state; for most moths and butterflies exist on love, and none take any more substantial nourishment than the honey of flowers.
The common fly has minute teeth. Many grubs live within trunks of trees, gnawing immense galleries, and subsisting on the wood. The termites, or "white ants," devour whole houses, leaving only a shell fair to all external appearance, but crumbling at a touch. "At Tonnay-Charente the termites, having gnawed away the props of a dining-room without its being perceived, the flooring collapsed during a party, and the entertainer and his guests sank through." The larva of the giant sirex gnaws burrows in lead.
Teeth of mollusks and articulates are usually horny—that is, hardened skin, like the crust of the lobster and beetle. Sometimes they
are calcareous and sometimes siliceous. Those of the vertebrates are complex in structure and substance, and are regarded as the only true teeth. They consist of dentine or ivory, of which there are several kinds. In mammals and the highest reptiles, the dentine is surrounded by a sort of bone called cement; and the surface exposed to wear is capped or otherwise protected by enamel, the hardest of animal tissues. Teeth do not belong to the bony skeleton, but are developed by the lining membrane of the mouth, which, like the lining of the whole food-canal, is only a continuation of the skin. Hence teeth are classed with other skin appendages, as the nails and hair.
The teeth of fishes are extremely various in number, form, structure, and method of attachment. They are almost entirely used for prehension, as nearly all fishes, like reptiles and birds, do not masticate but gulp down their food as quickly as possible. The exceptions, however, are interesting. Some rays and the cestracion of Australia have the jaws filled with teeth, flattened and joined together like blocks in a pavement, or like mosaic. These are used to crush sea-weed and mollusks. In a former geological age, this was the prevailing form of teeth in the whole order of sharks. The common carp has its teeth on the bones of the pharynx, and hence masticates its food in its throat. For this purpose the food is sent back after being swallowed. Some fishes are toothless, but most fishes have hundreds of teeth, frequently covering all parts of the mouth. The teeth of fishes and of reptiles are shed and replaced indefinitely.
Reptilian teeth present no great variety. Toads, tortoises, turtles, and some lizards are entirely destitute of teeth. Frogs have teeth in the upper jaw. Those of serpents assist in swallowing the huge prey. The poison-fangs of venomous species present a peculiar and complex modification. They are fastened to movable bones which are worked by the muscles moving the jaw. When the mouth is closed, the fangs lie on the gums; but in opening the mouth they are brought into a striking position. The muscles also press at the same time upon the glands which secrete the venom, and force the latter through a deep channel or canal in the fang.
Some birds of ancient times had true teeth placed in sockets in the jaws; but all modern birds depend wholly on the gizzard for mastication. This is literally a mill. It is formed of powerful muscles, has a horny lining, and pulverizes hard grain and indeed almost any substance by rubbing between the tough walls. To assist the grinding, the grain-eaters swallow gravel, bits of broken glass and crockery, metals, etc., which are pulverized in turn. One would suppose such
diet would injure the organ; but the tough lining yields without being cut. In birds the gizzard follows the stomach; that is to say, the food is not ground until after it is subjected to the gastric juice. This seems a reversal of the proper order, but the hard grain is more easily pulverized by rubbing after being softened by the solvent fluid. In other animals the gizzard, if a distinct cavity, usually precedes the true stomach. The power of this mill is proportional to the resistance of the food. Thus in flesh-eating birds the gizzard is weak.
Mastication is best exhibited in mammals and is almost entirely by means of teeth. Mammalian teeth are of three kinds: incisors or front cutting teeth; canines, which characterize flesh-eaters; and molars or masticators. They are placed in sockets in the bones, but always in a single row on the outer edge of the jaws, and are never renewed more than once.
The extreme numerical variation of mammalian teeth is found rather strangely in the same group, the order of whales. A river-dolphin of South America has the greatest number, two hundred and twenty-two; while the whalebone whale has no developed teeth, they being replaced with baleen-plates. The narwhal, or sea-unicorn, has, of two in embryo, but a single tooth developed. This, however, is remarkable as being the longest in the animal kingdom. It is the left upper canine, and except in rare cases possessed by the male alone. Instances are recorded where both teeth were developed. This ivory tusk points directly forward in line with the body, perfectly straight, and sometimes attains the length of ten feet. The rough surface is spirally grooved as if the tusk were twisted. Various unsatisfactory conjectures have been offered regarding the object of this strange development; but, beyond its evident use as a weapon, its purpose is still a mystery.
The whole group of ant-eaters, and the sloths and armadillos, are quite destitute of teeth, on which account they are called edentates. Many of them have no teeth whatever; and when teeth are present
they are limited to the back part of the jaws, without enamel and rootless—all of which features help to rank the edentates very low among mammals. In place of teeth, the duck-bill—the lowest of mammals—has four horny plates, two in each jaw.
The average number of mammalian teeth is thirty-two—possessed by man, apes, and ruminants. But, the hog is the happy possessor of the typical number, forty-four; which honor is also shared by the opossum and mole. Man is the only living animal with an unbroken succession of teeth, and having the canines of the same height as the others.
The incisors of rodents are very interesting. They are the only prehensile organs of the gnawing mammals, and are exposed to severe wear. But, as the enamel is thicker and the dentine harder in front, the abrasion constantly produces a chisel-edge admirably fitted for gnawing hard substances. They must, however, be kept of a certain length; and to supply the loss by abrasion they are continually growing out from the base, being supplied by a permanent pulp. Their length in the jaw is great, to insure solidity without actual union with the bone. The loss of an incisor results in the abnormal lengthening of the opposing one, which may finally interfere with mastication and cause starvation. Ruminants have no incisors in the upper jaw; the elephant has none in the lower jaw, but the tusks are upper incisors. An elephantine mammal of former geological time, the dinotherium, had incisor tusks in the lower jaw, pointing down and backward; while the extinct mastodon had tusks in both jaws.
The canines are intended for seizing and tearing prey and especially characterize carnivorous mammals. They are lacking in rodents and most herbivores, and are never more than four in number. In the apes they are very prominent; those of the gorilla nearly equaling a lion's in size. The tusks of the walrus are upper canines, as also are the terrible tusks of the wild hog. In the Malayan hog the upper
canines, instead of pointing downward, as would seem proper, grow upward through the integuments of the skull and curve backward, sometimes reaching the skull again. Their purpose is obscure. Although possessed in such degree by the male alone, the form precludes their efficient use as weapons. The lower canines also grow to enormous length and are directed outward, forming weapons which make the beast a formidable antagonist.
Herbivorous mammals have the molars flat on the grinding surface, and the enamel and cement disposed in plates and folds perpendicular to this face. Thus by the unequal wear of the tissues the acting surfaces are ridged and admirably adapted for grinding. They form an actual grist-mill, the stones of which never need any "picking." In fruit-eaters the crowns of the teeth are rounded. Insectivorous mammals have the teeth conical and fitting in opposing depressions. Those living on a mixed diet, as man, have the tubercles or cusps somewhat blunt, and suited either for crushing or cutting. In purely carnivorous mammals the molars have sharp edges fitted for cutting meat. They act like chopping-knives, or more accurately like scissors. Quite the only motion of the jaw is up and down, as flesh can not well be ground. Here we have a genuine "hash-mill." The backward and forward motion of the jaw prevails in the rodents. Other mammals have the lateral motion in a great degree, making three motions. All vertebrates have the jaws opening vertically, while articulates have horizontal jaws, and the sea-urchin has converging jaws.
The harmony and mutual dependence between all parts of an animal's body are not better shown than in the adaptation of the teeth of vertebrates to the kind of food, and their consequent agreement with the habits, structure, and form of the individual. A single tooth frequently reveals the entire character of its possessor. Indeed, no single organ is of greater zoölogical value. Not only does the student of living forms use the teeth as indicative of character and a means of classification, but their superior durability makes them of the very greatest importance in the study of ancient life.