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.
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.
Special Report: Sponge “Fisheries.” (Localities; how sponges are taken, cleaned, dried, shipped, and sold.)
CHAPTER IV
POLYPS (CUPLIKE ANIMALS)
The Hydra, or Fresh-Water Polyp
