thousands of cells which compose the many-celled animal body are differentiated and specialized; that is, have become changed or modified from the generalized primitive amœboid condition, so that each kind of cell is devoted to some special work or function and has a special structural character fitting it for its special function. In the Protozoan body the single cell can perform and does perform all the functions or processes necessary to the life of the animal. In the Metazoan body each cell performs, in co-operation with many other similar cells, some one special function or process. The total work of all the cells is the living of the animal. CHAPTER X HYDRA LABORATORY EXERCISE TECHNICAL NOTE.--_Hydra_ lives in fresh water, attached to stones, sticks, or decayed leaves. It can be found in most open fresh-water ponds not too stagnant, often attached to _Chara_. There are two species occurring commonly, _H. viridis_, the green _Hydra_, and _H. fuscus_, the brown or flesh-colored _Hydra_. Both are very small forms and have to be looked for carefully. Specimens should be brought to the laboratory, put into a large dish of water and left in the light. _Hydra_ is best studied alive. Place a living specimen attached to a bit of weed in a watch-crystal filled with water or on a slide with plenty of water and examine with the low power of the microscope. Note the cylindrical body (fig. 7, _A_, _B_) with its flat basal attachment and _radial tentacles_ (varying in number) which crown the upper end and surround the centrally located _mouth_. Note the movements of _Hydra_, its powers of contraction, and method of taking in food. TECHNICAL NOTE.--To feed _Hydra_, place very small "water-fleas" (_Daphnia_ sp.) in the water with it. Observe the method by which "water-fleas" are taken into the mouth. Food is caught on stinging cells (to be studied later) and conveyed to the mouth by the tentacles. Note that the cylindrical body encloses a cavity, the _digestive cavity_. How is this connected with the exterior? If _Hydra_ captures prey too large or is no longer hungry, the prey is released. [Illustration: FIG. 7.--A, _Hydra fusca_, with expanded body and a budding individual; B, _H. fusca_, contracted; C, _H. fusca_, part of outer surface of a tentacle, greatly magnified. (A and B drawn from live specimens, C, from a preparation) D, _Grantia_ sp. (a sponge), three individuals; E, _Grantia_ sp., longitudinal section; F, _Grantia_ sp., spicules. (D, E, and F drawn from preserved specimens.)] TECHNICAL NOTE.--Place small slips of paper on the slide near the _Hydra_, put cover-glass over the whole, and examine with the low power of the microscope.