An instructive experiment can be performed by cutting a _Hydra_ into two or more parts, when (usually) each of the various parts will develop into a complete _Hydræ_. This process may be called reproduction by fission, but it rarely occurs naturally. CHAPTER XI THE SIMPLEST MANY-CELLED ANIMALS =Cell differentiation and body organization in Hydra.=--From the examination of _Hydra_ we have learned that there are true many-celled animals which are much less complex in structure than the toad and crayfish. The body of _Hydra_, like the body of the toad, is composed of many cells, but these cells are of only a few different kinds; that is, show but little differentiation. There is relatively little division of the body into distinct organs. Still, certain parts of the body devote themselves principally to certain particular functions. Thus all the food is taken in through the single "mouth-opening" at the apical free end of the cylindrical body, and there are certain organs, the tentacles, whose special business or function it is to find and seize food and to convey it to the mouth. After the food is taken into the cylindrical body-cavity it is digested by special cells which line the cavity. Some of these cells are unusually large, and each contains one or more contractile vacuoles. From the free ends of these cells, the ends which are next to the body-cavity, project pseudopods or flagella. These protoplasmic processes are constantly changing their form and number. In addition to these large sub-amœboid cells there are, in this inner layer of cells lining the body-cavity, and especially abundant near the base or bottom of the cavity, many long, narrow, granular cells. These are gland-cells which secrete a digestive fluid. The food captured by the tentacles and taken in through the mouth-opening disintegrates in the body-cavity, or digestive cavity as it may be called. The digestive fluid secreted by the gland-cells acts upon it so that it becomes broken into small parts. These particles are seized by the projecting pseudopods of the sub-amœboid cells and taken into the body-protoplasm of these cells. The cells of the outer layer of the body do not take food directly, but receive nourishment only by means of and through the cells of the inner layer. The body-cavity of _Hydra_ is a very simple special organ of digestion.