CHAPTER VII

ADDITIONAL ANIMAL STUDIES

130. Sponges.

A. Porifera (sponges)

The sponges are animals more complex in structure than the Protozoa, for they are composed of many cells; nevertheless, they are comparatively simple in structure since they have no digestive, circulatory, respiratory, or nervous system, and therefore each cell has to carry on practically all the necessary nutritive functions.

Sponges differ largely in the kind of skeletons that they possess. In the common bath sponge (Fig. 123) this is composed of a tough, horny material. When

sponges are ready for market, only the horny skeleton remains, the living cells having been killed and removed. The sponge skeleton shows a large number of pores in the outer surface, and for this reason the name Porifera (Latin pore-bearing) is given to this group of animals. The pores lead into canals that run through the body, finally connecting with one or more larger central cavities that lead outward, usually at the top. In certain parts of these canals there are cells with cilia; their action causes water to rush into the canals through the pores, bringing food and oxygen to all the cells of which the sponge is

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composed. The wastes are forced out through the larger canals referred to above. Like the bath sponge, all other Porifera are stationary in their mature form.

B. Cælenterata

131. Hydra. A study of a fresh water cœlenterate known as hydra will give one a fair idea of the structure and adaptations of this group of animals. Hydra is a small animal found in fresh water attached to water plants, and sometimes to surfaces of stones or

FIG. 124. Longitudinal section of a hydra. (Hegner.)

other objects on the bottom. At the upper end of the tiny cylindrical column are threadlike bodies known as tentacles (Fig. 125, 1). If the animal is touched with a needle or pencil, it contracts its body and tentacles so much that it can scarcely be seen. But in a short time it expands again.

If the hydra happens to be hungry and some small form of animal

comes in contact with the waving tentacles, the hydra ejects microscopic threads from certain cells (nettling cells) in the tentacles. The animal thus attacked is benumbed, and the hydra then uses the tentacles to push its prey into a mouth opening in the center of the circular row of tentacles. The food is drawn into the inside of the column, which is simply a hollow tube (Fig. 124). Here certain cells secrete digestive fer

ments which dissolve the foods that the animal has eaten, and the indigestible matter is ejected from the mouth. The digested food is then absorbed by the cells lining the cavity. Since the animal is bathed outside and inside by water containing oxygen, the cells are able to absorb oxygen from the water and to give off carbon dioxid to the water. Hence no breathing organs are needed.

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FIG. 125.-The movements made by hydra in locomotion. (Jennings.)

It is evident that the tentacles with the nettling cells also serve to protect the hydra from too great familiarity on the part of visitors that might otherwise use it for food. When the hydra moves from one place to another, it bends over until the ends of the tentacles touch the surface on which it rests. The tentacles then adhere to this surface, the bottom of the column lets go, and the animal turns a somersault (Fig. 125) and lands on the lower part of the column; the process may then be again repeated.

Like the higher animals the hydra reproduces by means of eggs and sperms. But it also has another interesting way of producing new individuals. On the surface of the column one frequently sees little bunches. These are called buds (Fig. 124). They keep on growing outward till at last little tentacles and a mouth opening are

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formed at the tip of each. It is now evident that we are looking at a very tiny hydra. Finally the new individuals separate from the column and begin an independent life. This method of reproduction is known as budding.

132. Suggestions for the study of hydra. Laboratory study. Pupils should be supplied with living hydra if possible. The column and tentacles should be observed by the aid of a magnifier, described

FIG. 127.-Jellyfish. (Hargitt.)

and drawn. The animal should be touched and the action of the column and tentacles noted and described. If the hydra moves from place to place, the method of locomotion should also be described.

133. Relatives of hydra. Among the relatives of hydra are the corals (Fig. 126), sea-anemones, and jellyfish (Fig. 127). One form of ceral, the red coral, is of considerable economic importance. In all the corals the column secretes a mineral sub

stance within which the animal can withdraw when danger threatens. In the case of the red coral this material is horny. It is used for decoration, and some communities on the Mediterranean are devoted largely to the gathering of this coral, and to making it into various forms of jewelry.

134. Earthworm.

C. Annelida

The most common representative of the

annelida is the earthworm (Fig. 128). The general form of this animal is long and cylindrical. If one places an earthworm on the

FIG. 128. The earthworm. (Sedgwick and Wilson.)

ground, it will start to crawl away or bore into the soil. Observe that the end that is foremost is tapering. This is the anterior end. The opposite or posterior and is broader and considerably flattened. The part of the body on which the worm crawls is the ventral surface, which is somewhat flattened, while the dorsal surface is rounded. The whole body is composed of rings or segments. About one third of the distance from the anterior end of the worm several of the segments are usually somewhat enlarged and form the girdle.

At the anterior end toward the ventral surface, there is a small opening. This is the mouth, and through it the earthworm sucks in its food which consists not only of dirt, but of leaves of various kinds. Overhanging the mouth is a tiny projection, the lip. The animal has no special breathing organs. The skin, however, is permeated with capillaries, and thus serves as a breathing organ. Locomotion is brought about by alternately lengthening and then