THE HYDRA AS A SIMPLE METAZOAN 259 



Organization of the Endoderm. — The endodcrin, like the ecto- 

 derm, is composed mainly of large epitheUo-muscular cells (Fig. 

 122). In these, however, the basal processes run transversely 

 and thus provide a circular musculature for the body. The endo- 

 derm cells of the tentacles are without muscle processes, and hence 

 these organs have no mechanism within themselves by which they 

 may be elongated. They are extended by pressure of fluid forced 

 into them from the enteron of the body. Around the mouth and 

 at the base of each tentacle, the endodermal muscle processes are 

 especially developed as sphincters. In green hydras these large 

 endoderm cells are crowded with the bodies known as zoochlorelUr, 

 which are really a kind of unicellular green plant that has come to 

 be associated with the hydra. There are similar symbiotic plants 

 in the endoderm cells of some marine coelenterates, particularly the 

 corals. In the staining of hydras by students in the laboratory, the 

 nuclei of the zoochlorellse are sometimes recognizable. Being green 

 plants, they must take in carbon dioxide and liberate oxygen in 

 photosynthesis (c/. p. 170). In view of the needs of respiration, the 

 presence of such organisms within the c>i;oplasm of an animal's 

 cells would be advantageous. Likewise, the end products of oxida- 

 tion in the animal cell would be useful to the plant. A relation- 

 ship of this sort is termed symbiosis, or hving together with mutual 

 benefit, in contrast with the association known as parasitism, in 

 which the advantage is all on one side; or with commensalism, 

 in which the two organisms merely feed together without obvious 

 advantages or disadvantages. 



At a certain interval after the hydra has taken food, the endo- 

 derm cells are seen to be crowded with numerous particles of food 

 which are enclosed in vacuoles of digestion like those of a proto- 

 zoan such as amoeba. In starvation, these cells become highly 

 vacuolated; and when not overcrowded with food vacuoles they 

 are Ukely to contain one or more larger vacuoles like those found 

 in many plant cells, in addition to the spaces described (Fig. 122). 

 At their inner ends, where they are exposed to the digestive cavity, 

 the endoderm cells may have pseudopodia-Uke processes, by which 

 the food particles mentioned are engulfed; and also flagella, by 

 which the contents of the enteron are kept in motion in the absence 

 of movements of the hydra as a whole. Scattered among these 

 large endoderm cells are others having an elongated body and 

 enlarged at the end that is exposed to the enteron (Fig. 121). As 



