304 HYDRA CHAP. 



contain chromatophore-like bodies (chr) coloured green 

 by chlorophyll (p. 242), the function of which we have 

 already considered (p. 247). It has been proved, how- 

 ever, that these are not actual parts of the endoderm- 

 cells, but are distinct Sphserella-iike organisms known 

 as ZoochlorellcB, which are passed on from one generation 

 of the Hydra to another by entering its developing eggs. 

 Such an intimate 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 nitro- 

 genous 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 foodstuffs 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 endoderm 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 endo- 

 derm-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 the action of a digestive 



