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7. The waste products of this oxidation taken up by the blood must be got rid of; some from the lungs (carbon dioxide, water), some from the kidneys (water, urea, mainly), some from the skin (water, salines). (Respiration in part, excretion.)

The mechanism to accomplish all this in the lowest forms of life is exceedingly simple, a single cavity and surface performing all the functions. But in the majority of animals the apparatus is very complicated: there is a set of organs for the prehension of food; another for digestion; a third, for absorption; a fourth, for distribution; and a fifth, for purification.

CHAPTER X

THE FOOD OF ANIMALS

THE term food includes all substances which contribute to nutrition and furnish energy, whether they simply assist in the process, or are actually appropriated, and become tissue. With the food is usually combined more or less indigestible matter, which is separated in digestion.

Food is derived from the mineral, vegetable, and animal kingdoms. Water and salt, for example, are inorganic. The former is the most abundant, and a very essential article of food. Most of the lower forms of aquatic life seem to live by drinking: their real nourishment, however, is present in the water in the form of fine particles. The earthworm, some beetles, and certain savage tribes of men swallow earth; but this, likewise, is for the organic matter which the earth contains. As no animal is produced immediately from inorganic matter, so no animal can be sustained by it.

Nutritious or tissue-forming food comes from the organic world, and is albuminous, as the lean meat of animals and the gluten of wheat; oleaginous, as animal fat and vegetable oil; or saccharine, as starch and sugar. The first is the essential food stuff; no substance can serve permanently for food—that is, can permanently prevent loss of weight in the body unless it contains albuminous matter. As stated before, all the living tissues are albuminous, and therefore albuminous food is required to supply their waste. Albumen contains

nitrogen, which is necessary to the formation of tissue; fats and sugars are rich in carbon, and therefore serve to maintain the heat of the body, and to repair part of the waste of tissues. Many warm-blooded animals feed largely on farinaceous or starchy substances, which in digestion are converted into sugar. But any animal, of the higher orders certainly, whether herbivorous or carnivorous, would starve if fed on pure albumen, oil, Nature insists upon a mixed diet; and so we find in all the staple articles of food, as milk, meat, and bread, at least two of these principles present. As a rule, the nutritive principles in vegetables are less abundant than in animal food, and the indigestible residue is consequently greater. The nutriment in flesh increases as we ascend the animal scale; thus, oysters are less nourishing than fish; fish, less than fowl; and fowl, less than the flesh of quadrupeds.

or sugar.

Many animals, as most insects and mammals, live solely on vegetable food, and some species are restricted to particular plants, as the silkworm to the white mulberry. But the majority of animals feed on one another; such are hosts of the microscopic forms, and nearly all the radiated species, marine mollusks, crustaceans, beetles, flies, spiders, fishes, amphibians, reptiles, birds, and clawed quadrupeds.

A few, as man himself, are omnivorous, i.e., are maintained on a mixture of animal and vegetable food. The use of fire in the preparation of food is peculiar to man, who has been called "the cooking animal." A few of the strictly herbivorous and carnivorous animals have shown a capacity for changing their diet. Thus, the horse and cow may be brought to eat fish and flesh; the sea birds can be habituated to grain; cats are fond of alligator-pears; and dogs take naturally to the plantain. Certain animals, in passing from the young to the

mature state, make a remarkable change of food. Thus, the tadpole feeds upon vegetable matter; but the adult frog is carnivorous, living on insects, worms, and

crustaceans.

Many tribes, especially of reptiles and insects, are able to go without food for months, or even years. Insects in the larval, or caterpillar, state are very voracious; but upon reaching the perfect, or winged, state, they eat little some species taking no food at all, the mouth being actually closed. The males of some rotifers and other tribes take no food from the time of leaving the egg until death.

In general, the greater the facility with which an animal obtains its food, the more dependent is it upon a constant supply. Thus, carnivores endure abstinence better than herbivores, and wild animals than domesticated ones.

CHAPTER XI

HOW ANIMALS EAT

1. The Prehension of Food. - (1) Liquids. — The simplest method of taking nourishment, though not the method of the simplest animals, is by absorption through the skin. The tapeworm, for example, living in the intestine of its host, has neither mouth nor stomach, but absorbs the digested food with which its body is bathed (Fig. 37). Many other animals, especially insects, live upon liquid food, but obtain it by suction through a special orifice or tube. Thus, we find a mouth, or sucker, furnished with teeth for lancing the skin of animals, as in the leech; a bristlelike tube fitted for piercing, as in the mosquito; a sharp sucker armed with barbs, to fix it securely during the act of sucking, as in the louse; and a long, flexible proboscis, as in the butterfly (Fig. 221). Bees have a hairy, channeled tongue (Fig. 220), and flies have one terminating in a large, fleshy knob, with or without little "knives" at the base for cutting the skin (Fig. 222); both lap, rather than suck, their food.

Most animals drink by suction, as the ox; and a few by lapping, as the dog; the elephant pumps the water up with its trunk, and then pours it into its throat; and birds (excepting doves) fill the beak, and then, raising the head, allow the water to run down.

Many aquatic animals, whose food consists of small particles diffused through the water, have an apparatus for creating currents, so as to bring such particles within

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