The outer layer of cells serves as a sort of skin. Since so much of the sponge is in contact with water, most of the cells do their own breathing, or absorption of oxygen and giving off of carbon dioxide. _Nutriment_ is passed on from the surface cells to nourish the rest of the body. =Reproduction.=—Egg cells and sperm-cells are produced by certain cells along the canals. The egg cell, after it is fertilized by the sperm cell, begins to divide and form new cells, some of which possess cilia. The embryo sponge passes out at an osculum. By the vibration of the cilia, it swims about for a while. It afterwards settles down with the one end attached to the ocean floor and remains fixed for the rest of its life. The other end develops oscula. Some of the cilia continue to vibrate and create currents which bring food and oxygen. The _cilia_ in many species are found only in cavities called ciliated chambers. (Figs. 22, 32.) There are no _distinct organs_ in the sponge and there is very little _specialization_ of cells. The ciliated cells and the reproductive cells are the only specialized cells. The sponges were for a long time considered as colonies of separate one-celled animals classed as protozoans. They are, without doubt, many-celled animals. If a living sponge is cut into pieces, each piece will grow and form a complete sponge. [Illustration: FIG. 32.—Microscopic plan of ciliated chamber. Each cell lining the chamber has a nucleus, a whip-lash, and a collar around base of whip-lash. _Question_: State two uses of whip-lash. ] =That the sponge is not a colony of one-celled animals=, each like an amœba, but is a many-celled animal, will be realized by examining Fig. 32, which shows a bit of sponge highly magnified. A sponge may be conceived as having developed from a one-celled animal as follows: Several one-celled animals happened to live side by side; each possessed a threadlike flagellum (E, Fig. 32) or whip-lash for striking the water. By lashing the water, they caused a stronger current (Fig. 25) than protozoans living singly could cause. Thus they obtained more food and multiplied more rapidly than those living alone. The habit of working together left its impress on the cells and was transmitted by inheritance. Cell joined to cell formed a ring; ring joined to ring formed a tube which was still more effective than a ring in lashing the water into a current and bringing fresh food (particles of dead plants and animals) and oxygen. =Few animals eat sponges=; possibly because spicules, or fibres, are found throughout the flesh, or because the taste and the odour are unpleasant enough to protect them. Small animals sometimes crawl into sponges to hide. One sponge grows upon shells inhabited by hermit crabs. Moving of the shell from place to place is an advantage to the sponge, while the sponge conceals and thus protects the crab.