XVI. THE METAZOA - DIVISION OF LABOR

Problem XXVI. An introductory study of many-celled animals. (Laboratory Manual, Prob. XXVI.)

(a) Development.

(b) Sponges.

(c) The hydra.

(d) Development of tissues and organs. (e) Common functions.

Reproduction in Simple Plants. Although there are very many plants and animals so small and so simple as to be composed of but a single cell, by far the greater part of the animal and plant world is made up of individuals which are collections of cells living together.

In a simple plant like the pond scum, a string or filament of cells is formed by a single cell dividing crosswise, the two cells formed give rise each to two more, and eventually a long thread of cells results. Such growth of cells is asexual.

In some instances, however, a single cell was formed by the union of two cells, one from each of the adjoining filaments of the plant. Around this cell eventually a hard coat was formed, and the spore, as it was called, was thus protected from unfavorable changes in the surroundings. Later, when conditions became favorable for its germination, the spore might form a new filament of pond

scum.

In the seed plants, too, we found a little plant within the seed which, under favorable conditions, might give rise, through the rapid multiplication of the cells forming it, to a new plant. But the plant within the seed first arose from two cells, one of which, called a sperm, came from a pollen grain, the other of which, the egg, was found within the embryo sac of the ovary.

Reproduction in Simple Animals. In many-celled animals, as well as many-celled plants, the new animal is formed by the

union of a sperm and an egg cell. A common bath sponge, an earthworm, a fish, or a dog,- each and all of them begin life in precisely the same way. Animals which are thus composed of many cells are known as the Metazoa, as distinguished from the Protozoa, which are made of but a single cell.

Sexual Development of a Simple Animal. In a many-celled animal the life history begins with a single cell, the fertilized egg. This cell, as we remember, has been formed by the union of two other cells, a tiny (usually motile) cell, the sperm, and a large cell, the egg. After the egg is fertilized by a sperm cell, it splits into two, four, eight, and sixteen cells; as the number of cells increases, a hollow ball of cells called the blastula is formed; later this ball sinks

in on one side, and a double-walled cup of cells, now called a gastrula, results. Practically all animals pass through the above stages in their development from the egg, although these stages are often not plain to see because of the presence of food material (yolk) in the egg. In the sponge the gastrula, which swims by means of cilia, soon settles down, a skeleton is formed, other changes take place, and the sponge begins life as an animal attached to some support on the water. The early stages of life, when an animal is unlike the adult, are known as larval stages; the animal at this time being called a larva.

The young sponge consists of three layers of cells: those of the outside, developed from the outer layer of the gastrula, are called ectoderm; the inner layer, developed from the inner layer of the gastrula, the endoderm. A middle almost structureless layer, called the mesoderm, is also found. In higher animals this layer gives rise to muscles and parts of other internal structures.

The Structure of a Sponge. The simplest kind of a sponge has the form of an urn, attached at the lower end. A common sponge living in Long Island Sound is a tiny urn-shaped animal less than an inch in length. It has a skeleton made up of very tiny spicules of lime, of different shapes. Cut lengthwise, such an animal is seen to be hollow, its body wall being pierced with many tiny pores or holes. The bath sponge, the skeleton of which is made up of fibers of horn, or a variety known as the finger sponge, shows the pores even better than the smaller limy sponge. In a

A horny fiber sponge: IP, the incurrent pores; O, osculum. Notice that this sponge is made up of apparently several individuals. One fourth natural size.

a central cavity, which in turn opens by a larger hole, called the osculum, to the surrounding water.

A microscopic examination shows the pores of the sponge to be lined on the inside with cells having a collar of living matter surrounding a single long cilium or flagellum. The flagella, I lashing in one direction, set up a current of water toward the large inner cavity. This current bears food particles, tiny plants and animals, which are seized

Diagram of a simple sponge : I, inhalant openings; O, exhalant opening or

osculum.

and digested by the collared cells, these cells probably passing on the food to the other cells of the body. The jellylike middle layer of the body is composed of cells which secrete lime to form the spicules and the reproductive cells, eggs, and sperms.

.

The Hydra. Another very simple animal, which unlike the sponge lives in fresh water, is called the hydra. This little creature is shaped like a hollow cylinder with a circle of arms or tentacles at the free end. It is found attached to dead leaves, sticks, stones, or water weed in most fresh-water ponds. When disturbed they contract it into a tiny whitish ball little larger than the head of a pin. Expanded, it may stretch its tentacles in search of food

ba

digestive cavity

tentacle

Longitudinal section of a hydra: b, bud; ba, attached end; m, mouth; ov, ovary; sp, spermary holding sperm cells.

almost an inch from their point of attachment. The tentacles are provided with batteries of minute darts or stinging cells, by means of which prey is caught and killed. The outer layer of the animal serves for protection as well as movement and sensation, certain cells being fitted for each of those different purposes. Food Taking. The tentacles then reach out

like arms, grasp the food, and bend over with it toward the mouth. Certain cells lining the hollow body cavity pour out a fluid which aids in digest

ing the food. Other cells with long cilia circulate the food, while still other cells lining the cavity put out pseudopodia, which grasp and ingest the food particles. The tentacles are hollow, and the body cavity extends into them. The outer layer of the animal does not digest the food, but receives some of it already digested from the inner layer. This food passes from cell to cell, as in plants, by osmosis. The oxygen necessary to oxidize the food is passed through the body wall, seemingly at any point, for there are no organs for respiration (breathing).

1 A few sponges, for example, spongilla, live in fresh water.

Reproduction. The hydra reproduces itself either by budding or by the production of new animals by means of eggs and sperms, sexually. The bud appears on the body as a little knob, sometimes more than one coming out on the same hydra. At first the bud is part of the parent animal, the body cavity extending into it. After a short time (usually a few days) the young hydra separates from the old one and begins life anew. This is asexual reproduction.

The hydra also reproduces by eggs and sperms. These sperms are collected in little groups which usually appear near the free end of the animal, the egg cells developing near the base of the same hydra. Both eggs and sperms grow from the middle layer of the animal. The sperms, when ripe, are set free in the water; one of them unites with an egg, which is usually still attached to the body of the hydra, and development begins which results in the growth of a new hydra in a new locality.

The stages passed through in development resemble closely those already described on page 200, and it would not be hard to imagine the gastrula stage, turned upside down with a circle of tentacles at the open end. Our gastrula would then be a hydra. Division of Labor. If we compare the amoeba and the paramocium, we find the latter a more complex organism than the former. An amoeba may take in food through any part of the body; the paramœcium has a definite gullet; the amoeba may use any part of the body for locomotion; the paramecium has definite parts of the cell, the cilia, fitted for this work. Since the structure of the paramecium is more complex, we say that it is a "higher " animal. In the vorticella, a still more complex cell, part of the cell has grown out like a stalk, has become contractile, and acts and looks like muscle.

As we look higher in the scale of life, we invariably find that certain parts of a plant or animal are set apart to do certain work, and only that work. Just as in a community of people, there are some men who do rough manual work, others who are skilled workmen, some who are shopkeepers, and still others who are professional men, so among plants and animals, wherever collections of cells live together to form an organism, there is division of labor, some cells being fitted to do one kind of work, while others are fitted to do work of another sort.