MICROSYMBIOSIS 75 



Symbioses in the turbellarian worms have been described 

 in the following groups: Acoela, Rhaddocoela and Alloeo- 

 coela. The definitive symbionts in all cases are green and 

 brown algae. The thorough and extensive researches of 

 Keeble ('10) on the two marine species, Convoluta roscof- 

 fensis and C. paradoxa, represent one of the more extensive 

 and complete analysis of microsymbiosis that has come to 

 my notice. The observations and experiments of Keeble 

 were conducted over a period of ten years. His results 

 have been confirmed by other investigators. In his book 

 'Tlant-Animals, a Study in Symbiosis" he describes in 

 detail the habitat and habits of these two species. He 

 also describes a niunber of experiments determining the 

 reactions of the worms to various extrinsic influences. 

 These are valuable studies in animal behavior, but since 

 they have no direct bearing on our problem they will not 

 be further considered. 



Convoluta roscoffensis contains green algae and C. para^ 

 doxa harbors yellow-brown algae. In the adult condition 

 C. roscoffensis ingests no soHd food. Kept under abnormal 

 conditions, Keeble found that the animal may become 

 cannibahstic. C. paradoxa, on the other hand, is a vora- 

 cious feeder throughout hfe. The adult C. roscoffensis 

 depends upon the carbohydrate food suppUed by the sym- 

 biotic algae within its tissues. Obviously, the time must 

 arrive when there is nitrogen deficiency, and instead of 

 obtaining this from the outside, it digests the algal sym- 

 bionts that had previously nourished it. Soon after the 

 destruction of the symbionts, the animal dies. In the 

 early stages of development, C. roscoffensis contains no 

 algae, but feeds Uke most animals upon other animals 

 and plants. Then for a time it receives food from two 

 sources — from ingested plants and animals and from its 

 own green cells. Ultimately, it depends upon the algae 

 alone, and thereby brings about its own destruction. 



