tion. They can perform all the other life processes; they move the colony by lashing the water with their flagella; they take in food and assimilate it; they can feel. All the cells of the great colony, too, are intimately connected by means of protoplasmic threads. The protoplasm of one cell can mingle with that of another cell; food can go from cell to cell. The question whether the Volvox colony is a group of distinct organisms or is a single organism made up of cells among which there is a simple but obvious difference in structure and function; in other words, whether Volvox is a colony of one-celled animals, of Protozoa, or is a multicellular animal, one of the Metazoa (for so all the many-celled animals are called), is a difficult one to decide. Most zoologists class the Volvocina with the Protozoa-that is, they incline to consider Gonium, Pandorina, Volvox, and the other Volvocinæ as groups or colonies of one-celled animals.

20. Sponges. If the Volvocina be considered to belong to the Protozoa, the sponges are the simplest of all the many-celled animals. Sponges are not free-swimming animals, except for a short time in their young stage, but are fixed, like plants. They live attached to some solid substance on the sea bottom. They resemble plants, too, in the way in which the body is modified during growth by the environment. If the rock to which the young sponge is attached is rough and uneven, the body of the sponge will grow so as to fit the unevenness; if the rock surface is smooth, the body of the sponge will be more regular. Thus a sponge may be said to have no fixed shape of body; individuals of the same species of sponge differ much in form. The typical form of the sponges is that of a short cylinder or vase attached by one end and with the upper free end open (Fig. 17). Many individuals of one kind usually live together in a close group or colony, and they may be so attached to each other as to appear like a branching plant. This branching may be very diffuse, and the branches

may become so interwoven with each other as to form a very complex group. A sponge is composed of many cells. arranged in three layers-that is, the body of a sponge is a cylinder closed at one end whose wall is composed of three layers of cells. The outer layer of cells is called the ectoderm, and the cells composing it are flat and are all closely attached to each other. The inner layer is called the endoderm, and its cells are thicker than those of the ectoderm; they are also closely attached to each other. Sometimes they are provided with flagella like the flagellate Protozoa. The flagella are, however, not for the purpose of locomotion, but for creating currents in the water, which bathes the interior of the open cylindrical body. The middle layer, called the mesoderm, is composed of numerous separate cells lying in a jelly-like matrix. From these mesoderm cells fine needles or spicules of lime or silica often project out through the ectoderm. These minute sponge spicules are of a great variety of shapes, and they form a

sponges, Calcolynthus primigenius (after HAECKEL). A part of the outer wall is cut away to show the inside.

sort of skeleton for the support of FIG. 17.-One of the simplest the soft body mass. All over the outer surface of the body are scattered fine openings or pores, which lead through the walls of the body into the inner cavity. nected with the outside apical end of the body.

This cavity is of course also conby the large opening at the free or

There is hardly any differentiation of parts among the

sponges. As in the Protozoa, there are no special organs for the performance of special functions. The sponge feeds by creating, with its flagella, water currents which

flow in through the many fine pores of the body and out from the inner body cavity through the large opening at the free end of the body. These currents of water bear fine particles of organic matter which are taken up by the cells lining the pores and body cavity, and assimilated. There are no special organs of digestion. Each cell takes up food and digests it. The water currents also bring air to these same cells, and thus the sponge breathes. Although the sponge as a whole can not move, does not possess the power of locomotion, yet the protoplasm of the cells has the power of contracting, just as with the Protozoa, and the pores can be opened or closed by this cellular movement. Practically, thus, the only FIG. 18.-One of the simple sponges, movements the sponge can Prophysema primordiale (after HAECKEL). The body is represented

make are the movements made

as cut in two longitudinally. The by the individual cells.

large cells of the inner layer are the egg cells.

Reproduction is accomplished by a process of division, or by a process of conjugation and subsequent division. In its simplest way multiplication takes place by a group of cells separating from the body of the parent sponge,

becoming inclosed in a common capsular envelope, and by repeated division and consequent increase in number of cells becoming a new sponge. This is reproduction by "budding." The "buds," or small groups of cells which separate from the parent sponge, are called gemmules. Reproduction in the more complex way occurs as follows: Some of the free amoeboid cells of the mesoderm (the middle one of the three layers of the body wall) become enlarged and spherical in form. These are the egg cells. Other mesodermic cells divide into many small cells, which are oval with a long, tapering, tail-like projection. These cells are active, being able to swim by the lashing of the tapering tail. These are the fertilizing cells. The two kinds of reproductive cells may be formed in one sponge; if so, they are formed at different times. Or one sponge may produce only egg cells, another only fertilizing or, as they are called, sperm cells. Conjugation takes place between a sperm cell and an egg cell. That is, one of the small active sperm cells finds one of the large, spherical, inactive cells and penetrates into the protoplasm of its body. The two cells fuse and form a single cell, which may be called the fertilized or impregnated egg. This fertilized egg, remaining in the body mass of the parent sponge, divides repeatedly, the new cells formed by this division remaining together. The young or embryo sponge finally escapes from the body of the parent sponge, and lives for a short time as an active free-swimming animal. Its body consists of an oval mass of cells, of which those on one side are provided with cilia or swimming hairs. The cells of the body continue to divide and to grow, and the body shape gradually changes. The young sponge finally becomes attached to some rock, the body assumes the typical cylindrical shape, an aperture appears at the free end, and small perforations appear on the surface. The sponge becomes full grown.

It is unfortunate that most of us do not live on the

seashore, and hence can not observe the structure and life history of the living ocean sponges. There are, however, among the thousand and more kinds of sponges a few kinds which live in fresh water, and these are so widely spread over the earth that examples of them can be found. in almost any region. They belong to the genus Spongilla, and thirty or more species or kinds of Spongilla are known. In standing or slowly flowing water, Spongilla grows erect and branching, like a shrub or miniature tree; in swift water it grows low and spreading, forming a sort of mat over the surface to which it is attached. Reproduction takes place very actively by the process of budding. The budded-off gemmules are spherical in shape, and the cells of each gemmule are inclosed in an envelope composed of siliceous spicules of peculiar shape. These gemmules are formed in the body substance of the parent sponge toward the end of the year, and are set free by the decaying of that part of the body of the parent sponge in which they lie. They sink to the bottom of the pond or brook, and lie there dormant until the following spring. Then they develop rapidly by repeated division of the cells and growth.

It is impossible here to tell anything of the many and interesting kinds of sponges which inhabit the ocean. The "sponge" of the bathroom is simply the skeleton of a large sponge or group of sponges. The skeleton here is not composed of lime or silica, but of a tough, horny substance, which is secreted by cells of the mesodermal layer of the body wall of the sponge. This substance is called spongin, and is a substance allied to silk in its chemical composition. All the commercial sponges, the spongin skeletons, belong to one genus-Spongia. These sponges grow especially abundantly in the Mediterranean and Red Seas, and in the Atlantic Ocean off the Florida reefs, and on the shores of the Bahama Islands. The sponges are pulled up by divers, or by means of hooks or dredges. The