304 HYDRA CHAP. 



In Hydra viridis the endoderm-cells (Fig. 76, D) contain 

 chromatophore-like bodies (chr) coloured green by chloro- 

 phyll (p. 242), the function of which we have already 

 considered (p. 247). It has been proved, however, that 

 these are not actual parts of the endoderm-cells, but are 

 distinct Sphaerella-like organisms known as Zoochlorellce, 

 which are passed on from one generation of the Hydra to 

 another by entering its developing eggs. Such a living- 

 together of two organisms is known as symbiosis. It differs 

 essentially from parasitism (see p. 280), in which one 

 organism preys upon another, the host deriving no benefit 

 but only harm from the presence of the parasite. In 

 symbiosis, on the contrary, the two organisms are in a 

 condition of mutually beneficial partnership. The carbon 

 dioxide and nitrogenous waste given off by the cells of the 

 Hydra serve as a constant food-supply to the Zoochlorella : 

 at the same time the latter by decomposing the carbon 

 dioxide provides the Hydra with a constant supply of 

 oxygen, and also with two important food stuffs starch 

 and proteids, which, after solution, diffuse -from the proto- 

 plasm of the Zoochlorella into that of the endoderm-cells. 

 The latter may therefore be said to keep the Zoochlorellae 

 constantly manured, while the Zoochlorellae in return supply 

 them with oxygen and ready-digested food. In the endo- 

 derm of H. fusca bodies of an orange or brown colour are 

 present which are devoid of chlorophyll. 



Muscle-processes also exist in connection with the endoderm-cells, 

 and they are said to take a transverse or circular direction, i.e., at 

 right angles to the similar processes of the ectoderm cells. 



When a water-flea or other minute organism is swallowed 

 by a Hydra, it undergoes a gradual process of disintegration. 

 The process is begun by a solution of the soft parts due to 



