118 PLANT-ANIMALS [CH. 



kept under observation for nearly three weeks, only 

 five animals contained green cells. In another case, 

 not a single animal of a total of forty-seven was 

 found to contain any trace of green cells or colourless 

 precursors. 



Further, on transferring such uninfected animals 

 to ordinary, unfiltered sea- water, they became uni- 

 formly green in the course of one or two days. 



We conclude therefore that the green cells of 

 C. roscoffensis are alga3 ; that the species to which 

 they belong exists as a free-living, independent, 

 marine plant; that this alga has a colourless stage, 

 as well as a green stage, in its life-history ; that the 

 alga lives on the egg-capsule as well as in sea-water ; 

 that it is ingested with the food, and, resisting 

 digestion, is planted in the body where it increases 

 and multiplies and forms the green tissue of adult 

 C. roscoffensis. 



The questions remain : What is this alga and what 

 does it look like in its free stage? 



All sorts of attempts, some ludicrous in their 

 extravagance, were made to isolate the infecting 

 organism ; whilst all the time it was lying under the 

 eye. None of the attempts succeeded, and, during the 

 winter, when experimental work could not be carried 

 on, there was nothing to do but to contemplate rue- 

 fully the note-books recording the failures. But some 

 wise person once observed, " You learn to play cricket 



