in] GREEN CELLS OF CONVOLUTA 97 



and, after giving their coloured cells a respite 

 of some weeks, they turn on these algal cells and 

 digest them. In C. paradoxa, this raiding of the 

 coloured cells occurs only under special, artificial 

 conditions ; as, for instance, during prolonged dark- 

 ness. But, in the case of C. roscoffensis, it is a regular 

 procedure with animals which have reached a certain 

 age. Nor is the reason for this difference of be- 

 haviour between the two plant-animals far to seek. 

 Whereas C. paradoxa retains its habit of ingesting 

 solid food and looks to its yellow-brown cells for sup- 

 plementary supplies only, C. roscoffensis, at a certain 

 stage, shuts its mouth and cultivates its garden of 

 green cells. Now, inasmuch as hunger cell-hunger 

 may be due to one or more of many different 

 lacks, lack of carbohydrate, lack of nitrogenous food- 

 substances, or of mineral compounds, it is bound to 

 happen sooner or later that the animal part of C. 

 roscoffensis, in its phase of total abstinence from 

 food, will feel the pinch of one kind of hunger or 

 another. Goaded by this all-powerful stimulus it 

 turns upon its green cells, and, biting the hand that 

 fed it, seeks, by devouring them, to satisfy its cravings 

 for some special food-substances. 



To the question what particular kind of hunger 

 is it that drives the animal to devour its plant-like 

 cells, we shall address ourselves, after we have in- 

 vestigated, in the next chapter, the origin of these 



K. 7 



