UNICELLULAR FORMS 91 



Specialization is confined to individual cells in all cases; nowhere 

 among such colonial forms is a tissue of like cells found. The devel- 

 opment of tissues and complex organs composed of tissues is found 

 only in the multicellular forms. 



Comparisons with the Simplest of the Metazoa. While 

 colonial protozoons as exemplified by volvox anticipate to a slight 

 extent some of the relations that exist between the cells and tissues 

 of a metazoon body, there is a rather wide gap between such com- 

 paratively simple colonies and the body of the most simple meta- 

 zoon. It is said that a volvox colony may consist of as many as 

 1,200 cells; the most simple adult metazoon body usually contains 

 many more than that number. Moreover, the division of labor 

 within a protozoon colony is more or less temporary, at least as 

 regards the reproductive process, while the various tissue cells within 

 the metazoon body have functions that are stable and more or less 

 permanent. Perhaps the most simple metazoon that may come to 

 the attention of the reader is a small animal found in fresh water 

 that, when observed casually, very much resembles a plant. The 

 fresh water hydra, as it is called, is a vase-shaped organism, never 

 longer than an inch and often much shorter, usually observed at- 

 tached by its narrow or basal end to a stem or the walls of an 

 aquarium, its opened upper end surrounded by tentacles that ap- 

 pear to be, and actually are extended to grasp any small animal 

 that may come within reach. When seen under the microscope, it is 

 found to be composed of two layers of cells, that is, two chief tis- 

 sues, the cells of which are distinctly different in appearance and 

 shape (Figs. 33, 79). The inner layer is assigned the function of 

 digesting all the food for the entire animal; this is done by engulfing 

 particles of food within the cells, as do the Protozoa, and also by 

 the enzymes that are freed from these cells into the central or 

 gastric cavity. The cells of the outer layer have taken over several 

 functions. Some cells are quite contractile and may therefore be 

 said to be muscular in their function. Others (Fig. 139) are known 



