CH. iv] GREEN CELLS OF CONVOLUTA 101 



elements are invariably present. In other animals, 

 the coloured cells may occur in some individuals, 

 but not in others. The former, general association 

 we may call obligate, and the latter, occasional 

 association, facultative. 



Hydra viridis, Convoluta roscoffensis and C. 

 paradoxa are examples of organisms in which the 

 association is obligate. 



Facultative association may take one of two 

 forms. Either some specimens living in a given 

 region may possess coloured cells, whilst other speci- 

 mens of the same region lack them, or a given 

 species may consist, in one part of its range, of 

 individuals all of which contain coloured cells, and, 

 in another part of its range, of individuals none of 

 which possess them. For example, Noctiluca is colour- 

 less in the North Atlantic, but green in the Indian 

 Ocean. British Alcyonium have no chlorophyll- 

 containing cells, whereas the nearly allied Alcyonium 

 ceylonicum possesses them. It seems probable and 

 this is a point of which we shall make use presently- 

 that association between animal and plant-like cells 

 is commoner in the warmer than in the colder seas. 



The problem of the origin and nature of the green, 

 yellow and brown cells which occur in animals has 

 engaged, from time to time, the attention of zoologists. 

 Long ago the name Zoochlorella was given to the green 

 cell and Zooxanthella to the brown or yellow-brown 



