THE GREEN CELLS OF CONVOLUTA ROSCOFFENSIS. 207 



iiitrogeu linnger, ifc turns upon the green cells and raids such 

 stores as they contain by digesting them. These stores are 

 but limited^ and when exhausted, death, both of the animal 

 and of its green cells, inevitably follows. 



The election by the infecting alga of the egg-capsules and 

 the surface of the animal as a normal habitat may also be 

 regarded as a symptom of nitrogen hunger; so, too, is the 

 ultimate digestion of the green cells by the animal. 



Taking the broadest view of the whole relationship, includ- 

 ing therein, not only the green cells inhabiting the body of 

 Convoluta, but also the free green cells living on soluble 

 organic waste contained in the egg-capsules, we may classify 

 it as a symbiotic relationship, for it is now only by reason of its 

 infection by tlie green cells that Convoluta roscof f eusis, 

 the species, persists, and though the green cells which enter 

 the animal never escape alive, this is, as it were, but the price 

 which the species has to pay for its lodging. 



But if we con6ne our attention to a green Convoluta, not 

 looking beyond the association of green cells and animal, then 

 the relation constitutes a case of parasitism, the host being the 

 green cells and the parasite the free-living animal. 



The extraordinary restriction of the range of the species 

 C. roscoffensis we may regard as the result of this peculiar 

 and economically unsound attempt on its part to solve the 

 " nitrogen question." 



Summary of Section VI. 



(1) Convoluta exhibits four phases of nutrition, passing 

 from the typically animal to the completely parasitic. 



(2) The infecting alga shows specialisation in the direction 

 of saprophytism. 



(3) This habit enables the green cells to utilise the products 

 of the animal's nitrogenous metabolism, and so to develop 

 rapidly within the body, where they serve as an excretory 

 system to the animal. 



(4) The habit of the infecting alga, in its free state, of fre- 



