Some of the silicious sponges have skeletons that resemble spun glass in their delicacy. Flint is chemically nearly the same as glass. The skeleton shown in Fig. [29] is that of a glass sponge which lives near the Philippine Islands.
The horny sponges do not have spicules in their skeletons, as the flinty and limy sponges have, but the skeleton is composed of interweaving fibres of spongin, a durable substance of the same chemical nature as silk (Figs. [30] and [31]).
The limy sponges have skeletons made of numerous spicules of lime. The three-rayed spicule is the commonest form.
Fig. 30.—A horny sponge.
Fig. 31.—Section of horny sponge.
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 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.