THE GREEN CELLS OF CONVOLUTA KOSCOFFENSIS. 169 



Cassiopeia^ a medusa of warm seas, has adopted, as a conse- 

 quence of the association with itself of green cells, a posture 

 the reverse of that maintained by its allies. On certain reefs 

 it may be seen fixed by its aboral surface, exposing its oral 

 arms, and so the coloured cells contained therein to the light. 



A^arious reef corals, richly provided with zoochlorellffi, 

 show marked decrease in the size of their tentacles ; in this 

 respect, and also in the number and degree of development 

 of their digestive filaments, they present a sharp contrast 

 with their more freely feeding allies (Pratt, 1905). Facts 

 such as these lead naturally to the hypothesis that the 

 association of animal and green cell results in a supply of 

 nutriment to the former from the latter. 



Admitting, as it is reasonable to admit, this trophic hypo- 

 thesis, it still remains to inquire — as we do in the case of 

 Convoluta ro scoff ens is in the following sections — what 

 food-substances are transferred from green cell to animal and 

 how this transference is effected. 



It is important to know how this hypothesis stands with 

 respect to such cases of voracious feeders as anemones. 

 Hydra, Convoluta paradoxa, and many green rliab- 

 docoels. Here it is clear that the animal is not subdued to 

 the working of the "plant" within it. 



It may also be asked how this at present vague hypothesis 

 meets the cases of facultative association. One of the two 

 main purposes of this paper is to put forward a supplemen- 

 tary hypothesis (Section VI, page 205), which we think 

 throws new light on the significance of chlorella? and xanthell^ 

 in animals. The other purpose of this paper is to provide 

 absolute proof of the source of origin of the green cells in 

 Convoluta roscof fensis. 



The view generally held, that the green or brown cor- 

 puscles of animals are of algal nature, is based rather on 

 probability than on certain evidence. 



The usual arguments in favour of this view are as follows : 



The corpuscles in question have been shown in certain 

 cases to be capable of photosynthesis. In the light, gases 



