CHAPTER XVI

ANIMÁLS

332. What is an Animal? We have already studied something of the relation between animals and plants. Plants (those containing chlorophyll; cf. § 310) change carbon dioxide, water, and the minerals of the soil into living material, which serves as food for the animal world. In taking carbon dioxide from the air, plants remove a waste product of animals, and restore oxygen in its place. Thus they keep the composition of the air the same, year after year. It was once supposed that plants did not produce carbon dioxide. This is a mistake. Plants give off carbon dioxide in their respiration, just as animals do; but green plants take up much more than they give off. Hence they seem not to produce any.

In general, we may say that an animal is an organism that cannot build up its food out of mineral matter, while a plant can. Plants having no chlorophyll (fungi) are exceptions to this statement (cf. § 324). The lowest forms of animals and plants are really so much alike that it is often hard to say to which class a given creature belongs. The fact that the higher animals can move about freely is not a real difference; for many animals are attached to some fixed place, while some plants have very rapid movements.

333. One-Celled Animals. In stagnant water or in mud there is often found a bit of jelly-like substance that

ONE-CELLED ANIMALS

Nucleus

FIG. 253.

313

An Ameba, magnified.

shows, under the microscope, a peculiar power of changing its shape, and of moving about. This is one of the lowest animals; it is called an ameba (Fig. 253), from a Greek word meaning "to change." The jelly-like creature consists of a cell of protoplasm (cf. § 323). At its center the cell is grainy; this region. is the nucleus. The ameba thus consists of one cell, like protococcus; but it has no chlorophyll. It has no organs, either. It moves by extending the cell wall at any region into a projecting lobe; the cell contents then flow into the lobe. These lobes are called "false feet." The ameba has no mouth; it feeds by extending two lobes around an article of food, and then withdrawing the cell wall. The protoplasm simply envelops the food. The ameba excretes (throws out) waste parts of its food merely by moving away from them. When food is thus passed through the cell wall, no opening is left; just as there is no break in the tough, though fragile, film of a soap bubble when a needle is put through it.

The reproduction of the ameba is also of the simplest sort. The nucleus of the cell divides itself into two parts, half of the protoplasm groups itself about each nucleus, and at the line thus formed the cell divides; there are now two amebas instead of one.

A creature like the ameba forms a part of our own blood (cf. § 378). If you will examine a drop of blood, under the microscope, you will see that it contains both red and colorless bodies. These are called corpuscles, that is, "little bodies." The colorless ones, called white

corpuscles, have the form and the simple structure of the ameba; but they are not so active.

Nucleus

A more highly developed one-celled animal is called the "slipper animalcule," from its shape (Fig. 254). Animalcule (pronounced, ǎn-i-măl-kül) means little animal." The scientific name of the creature is paramecium. In this case the cell has

FIG. 254.

Cilia

Mouth

Slipper Animalcule (Para

mecium), magnified. View of the Under Side.

a permanent place

for feeding, called

the mouth, and the
mouth extends in-

ward, forming a gullet. The creature has a permanent shape, because its cell wall is more stiff than that of the ameba. It swims about by waving the thread-like extensions (cilia; cf. § 324) of its cell wall.

A third one-celled animal is called vorticella, or the "bell animalcule" (Fig. 255). Its shape is like that of a bell with a long handle. It is called vor

Tentacles

Mouth

Cavity

肚

Bud

Ectoderm

Endoderm

FIG. 256. Hydra, or Polyp.
Exterior View and Cross
Section. Magnified.

FIG. 255. Bell Animalcule (Vorticella), magnified.

ticella because, by waving the cilia at its mouth, it produces a vortex, or whirlpool, by which food is carried into the cell. The cell wall is extended to form a "stalk," by which the creature attaches itself to weeds, or to sticks and stones in the bottom of ponds.

334. Simple Many-Celled Animals; Hydra. If a water plant is placed in a glass vessel of water, a hydra (Fig. 256) is often found attached to the plant or to the sides of the vessel. It is a brown or greenish creature half an inch long, or less.

When it is not disturbed, the hydra extends several thread-like "arms," called tentacles, about in the water.

SIMPLE, MANY-CELLED ANIMALS; HYDRA

315

If tiny bits of meat are dropped near them, the tentacles direct the meat to the mouth. But food does not go directly into the cell, as it does in one-celled animals. We can see that this is not necessary, for the hydra's body and tentacles are made up of two rows of cells. The inner cells have the duty of digesting food, while the outer cells serve to protect them. The digested food is passed through the cell walls, from the inner cells to the outer ones. The tentacles do not simply push food toward the mouth, but some of their outer cells have stinging weapons for paralyzing the hydra's prey.

Often the hydra may be seen budding, as yeast does (cf. Fig. 103, § 129). The young hydra remains attached to the older one for a time, and then separates from it. If a hydra is cut in two, each half produces the necessary cells, and forms a new creature. The hydra is also reproduced by a sexual method. There are not two sexes, but some of the cells produce egg-like bodies, and other cells on the same hydra produce swimming cells that fertilize the egg-like cells. When the two kinds of cells unite, they form an embryo hydra. This falls to the bottom of the water, and in time develops into a full grown animal.

FIG. 257.

A Coral

Colony.

More highly developed forms of the hydra have a great number of animals in a colony (Fig. 257). The colony has root-like "holdfasts," and attaches itself to rocks. Hydra-like animals are called polyps, from words meaning "many feet." is a common form. Each coral polyp takes limestone from sea water, and forms a hard internal structure (coral), which remains when the

The coral polyp

limestone taken

creature dies. The accumulations of coral rock from sea water in past ages are enormous in extent (cf. § 132). Sponges (Fig. 258) are many-celled animals less well developed than hydras. The individual creature has two cell layers separated by a gelatinous layer. The gelatinous layer produces the tough, horny material that we call "sponge." The animal is very soft, and the horny structure keeps its body cavity open. The cells have cilia, which produce incoming water currents bearing food. The outer openings of the body cavities form the tiny pores of the "sponge." Our sponges therefore represent colonies of the sponge animal with the horny material joined in a large mass. Some sponges form skeletons of limestone or of silica (the substance of powdered quartz), instead of horny structures.

335. Starfishes.- One of the interesting pastimes at the seashore is watching the movements and other activities of the starfishes (Fig. 259). The starfishes belong in a class with sea-urchins, sea-cucumbers, etc. This class is more highly developed than sponges and hydras.

Madreporite.

*

FIG. 259. Starfish.

The grown starfish is arranged on a plan of 5. The central region is round, and the "points of the star," or "arms," branch out from it. The covering of the animal contains hard limestone knobs with projecting spines. The body has a digestive system that is distinct from the interior cavity (cf. § 334); it has also a simple nervous system, and the beginnings of a blood system.