102 PLANT-ANIMALS [CH. 



cell. Since, however, these names are applied, re- 

 spectively, to any green and any brown plant-like 

 cell which occur in any animal their value is but 

 limited. 



That Zoochlorellse and Zooxanthellae are plant- 

 like cells is undisputed. They contain chlorophyll, 

 decompose carbon-dioxide with evolution of oxygen, 

 may, in the case of Zoochlorellse, contain starch : a 

 substance for the manufacture of which plants and 

 not animals possess the secret. Further, in some 

 cases, at all events, the coloured cells possess a wall 

 of cellulose, another substance the formation of 

 which is confined exclusively or almost exclusively 

 to members of the vegetable kingdom. 



Beside one or more chloroplasts, a nucleus and 

 a pyrenoid, the coloured cells have been shown in 

 some cases to contain a small, bright red body known 

 as an eye-spot (Fig. 21, p. 123). In free-living, uni- 

 cellular algae, the eye-spot serves the purpose of light- 

 perception and thus is part of the nervous machinery 

 for the performance of phototropic movements. Hence 

 its occurrence in green cells imprisoned in the bodies 

 of animals may be regarded as a strong indication 

 that the green cell which possesses it had once a 

 free-living existence. 



Nevertheless, though such facts as these lend 

 powerful support to the hypothesis that Zoochlorellee 

 and Zooxanthellee are algal cells which have aban- 



