THE GREEN CELLS OF CONVOLUTA ROSCOFFENSIS. 205 



(4) Old green Convoluta; the only term that can be applied 

 is autotrophic; the green animal behaves to its green cells 

 just as any animal or plant may behave to any reserves of 

 food-material ; it digests them. 



If we consider the same facts from the point of view of the 

 animal Convoluta ; then stages two, three, and four represent 

 the progressive parasitism of the animal on its contained green 

 cells. 



We have next to consider the sources of the supplies of 

 nitrogen compounds to the green cells and to the animal. 



The facts already described, proving that the infecting 

 alga thrives better on organic than on inorganic nitrogen 

 compounds suggest that the habit of this alga of settling- 

 down on the egg-capsules of Convoluta originated in obedience 

 to these nitrogen-requirements. 



The following considerations and observations lend sup- 

 port to this view. Whereas most Turbellarians have a well- 

 developed excretory system of flame-cells, Convoluta 

 roscoffensis has none. Nor do we find in this species 

 those granular accumulations, often appearing as localised 

 refractive bands or patches, which are constant features in 

 allied species and which von Graff regards as being of an 

 excretory nature. The fruitlessness of our prolonged search 

 for flame-cells justifies us in holding that none exist in 

 Convoluta roscoffensis. Moreover, green Couvolutas 

 show no signs of any excretory substances. The green cells 

 we know are capable of utilising such excretory nitrogen 

 compounds as uric acid and urea. It seems, therefore, pro- 

 bable in a high degree that the green cells of Convoluta are 

 its excretory system. 



The following observations lend a certain measure of 

 support to this hypothesis. If larval Convolutas are protected 

 from infection and kept without food, their large stores of 

 reserves gradually disappear, and vacuoles charged with long 

 acicular crystalline bodies make their appearance. The 

 numbers of these vacuoles and of their contained crystals 

 increase till they form one of the most striking features of the 



