Page:First course in biology (IA firstcourseinbio00bailrich).pdf/257

 without doubt, many-celled animals. If a living sponge is cut into pieces, each piece will grow and form a complete sponge.

That the sponge is not a colony of one-celled animals, each like an ameba, 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 thread-like 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.

—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.

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.

No animals eat sponges; possibly because spicules, or fibers, are found throughout the flesh, or because the taste and odor is unpleasant enough to protect them. Small animals sometimes crawl into them to hide. One species grows upon shells inhabited by hermit crabs. Moving of the shell from place to place is an advantage to them, while they conceal the crab and thus protect it.

Special Report: Sponge "Fisheries." (Localities; how sponges are taken, cleaned, dried, shipped, and sold.)