In handling a sponge the most indifferent person has not failed to observe two peculiarities, one that the sponge is soft, another that it is filled with holes, small and large, and has a marvelous faculty of retaining water, in which property lies its value to man. Now if we take a sponge and cut down through it, making a section, we shall find that these holes are nothing more or less than doors or mouths which lead into the interior of the sponge. If we are so fortunate as to have a live sponge to study, we shall see that water is being forced through all the small pores and out into the larger ones; and if we could examine the water, we should see that the water which passed in, is laden with living creatures which have been described in a previous chapter, while the water which is discharged contains little or no living matter. The reader will have suspected what this in-going and out-coming is. It is the operation of eating on the part of the sponge, which, while it looks very much like a plant and appears to be growing from the ground, is an animal, or a community of many cells--a many-celled animal. [Illustration: FIG. 15.--Flint spicules highly magnified.] In our section of a sponge we may follow the winding channels which connect one part of the sponge with the other, and we see that the body is a mass of fibers made up of curious and beautifully shaped objects called spicules (Fig. 15). The spicules are the bones of sponges, the hard portion, the framework. In the sponge we find here and there little oval rooms, and in these are packed, side by side, minute objects with tails (Fig. 16). Each little cell has its tail upon the outside, which is really a whip or lash, used for capturing passing food. In these rooms the cells congregate and are fed by the constant onward flow of water laden with food. The food is in the shape of minute animals and plants which these little whips catch as they pass by. The whips have another purpose; their constant motion serves to force the water along through the canals, carrying air as well as food. [Illustration: FIG. 16.--Parts of a sponge (_Grantia_): _B_, cross section showing pores leading into tubes _C´_; _C_, enlarged tube; _D_, cells magnified.] Some of the sponges have very singular shapes. One is called the finger sponge, and often takes the form of a hand. Others are very long and slender (Fig. 17). Some are perfectly round; others creep over stones and form a brilliant red matting, a charming object in the water.