=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. [Illustration: 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.] 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.) CHAPTER IV POLYPS (CUPLIKE ANIMALS) THE HYDRA, OR FRESH WATER POLYP [Illustration: FIG. 33.--A HYDRA.] SUGGESTIONS.--Except in the drier regions of the United States, the hydra can usually be found by careful search in fresh water ponds not too stagnant. It is found attached to stones, sticks, or leaves, and has a slender, cylindrical body from a quarter to half an inch long, varying in thickness from that of a fine needle to that of a common pin. The green hydra and the brown hydra, both very small, are common species, though hydras are often white or colorless. They should be kept in a large glass dish filled with water. They may be distinguished by the naked eye but are not studied satisfactorily without a magnifying glass or microscope. Place a living specimen attached to a bit of wood in a watch crystal filled with water, or on a hollowed slip, or on a slip with a bit of weed to support the cover glass, and examine with hand lens or lowest power of microscope. Prepared microscopical sections, both transverse and longitudinal, may be bought of dealers in microscopic supplies. One is shown in Fig. 39.