168 F. KEEBLE AND F. W. GAMBLE. 



some animals they are present constantly, in others they occur 

 sporadically. 



Green, yellow, or brown cells have been described in 

 representatives of every division of free-living Protozoa, in 

 certain sponges, in most anthozoau Coelenterates, in a few 

 Hydrozoa and Scyphozoa, and in accelous and rhabdocoelous 

 Tnrbellaria. Their occurrence in the higher groups is rare — 

 6. g. Zoobothrium (Polyzoa), Elj'sia (MoUusca), and Echino- 

 cardium (Echinoderms). 



The association is obligate in Convoluta par ad ox a, 

 C. roscof fensis, and in Hydra viridis; it is facultative 

 in the Protozoa, Anthozoa, and rhabdococl Tnrbellaria. 



In the former cases every individual of a species possesses 

 green or yellow or brown cells ; in the latter cases only 

 certain individuals contain them. 



Facultative association may exhibit itself in another manner: 

 in one part of its range all the individuals of a species may 

 exhibit the association, in another part the coloured cells 

 may be absent from all the individuals. Thus Noctiluca is 

 colourless in the North Atlantic and green in the Indian 

 Ocean. British Alcyonium have no zoochlorell^e, whereas the 

 closely allied A. ceylonicum possesses them (Pratt, 1905). 

 It seems probable, indeed, that the maximum development 

 of these associations occurs in the warmer seas. According 

 as the association is facultative or obligate, it gives rise to a 

 less or greater modification in the behaviour and in the 

 structure of the animal. 



Convoluta roscoffensis exhibits in a striking degree 

 such modifications (Gramble and Keeble, 1903). It lives 

 gregariously at mid-tide level on the beach and exposes 

 itself to the bright light of mid-day. Its colourless young 

 are as strongly phototropic as the green adults. With the 

 advent and multiplication of the green cells it ceases to ingest 

 solid food. Structural changes described in the body of this 

 paper also follow consequent upon the association of animal 

 and green cell. 



Similar phenomena arc exhibited by other animals. Thus 



