[Illustration: FIG. 29.--Skeleton of a glass sponge.] The _horny sponges_ do not have spicules in their skeletons, as the flinty and limy sponges have, but the skeleton is composed of interweaving fibers of _spongin_, a durable substance of the same chemical nature as silk (Figs. 30 and 31). [Illustration: FIG. 30.--A horny sponge.] [Illustration: FIG. 31.--Section of horny sponge.] The _limy sponges_ have skeletons made of numerous spicules of lime. The three-rayed spicule is the commonest form. The commercial sponge, seen _as it grows in the ocean_, appears as a roundish mass with a smooth, dark exterior, and having about the consistency of beef liver. Several large openings (oscula), from which the water flows, are visible on the upper surface. Smaller holes (inhalent pores--many of them so small as to be indistinguishable) are on the sides. If the sponge is disturbed, the smaller holes, and perhaps the larger ones, will close. 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 dioxid. _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.