76 SYMBIONTICISM AND THE ORIGIN OF SPECIES 



The two worms are remarkable among the turbellarians 

 in possessing no excretory system. The nitrogenous waste 

 products are stored in the tissues of the animal. In the 

 cells of young worms which do not yet possess any algal 

 symbionts, crystalline bodies can be seen. These cells 

 remind one of the ''urate cells" of insects. After the algae 

 have invaded the tissues of the worms, these crystals dis- 

 appear, and in adult life they are absent. ''Comparative 

 cultures of the free stage of the infecting organism have 

 demonstrated that the algae flourish better when suppUed 

 with nitrogen in the form of uric acid than when it is sup- 

 plied with a nitrate (potassium nitrate)." 



The transmission of the symbionts in the reproduction 

 of the host does not follow in the manner that one might 

 expect in such cases of obhgate symbiosis. The egg cell 

 itself does not contain any symbionts, but the egg capsule 

 was found to harbor the algae. It is only in a later develop- 

 mental stage that the offspring become infected. 



The host in one of these symbiotic associations (C. roscof- 

 fensis) exhibits definite physiological and morphological 

 responses to the invasion of the symbiont. The cessation 

 of food ingestion from the outside apparently develops 

 into a fixed physiological habit. The failure in the develop- 

 ment of an excretor}^ system appears to be an evident mor- 

 phologic variation associated with the anticipated presence 

 of uric acid-consuming symbionts. Keeble says "the con- 

 clusion forces itself upon us that the green and yellow- 

 brown cells in the bodies of their respective hosts obtain 

 access to and utihze the stores of waste nitrogen compounds 

 accumulated therein. Or, to put the same idea in another 

 way, green cells and yellow-brown cells constitute the 

 excretory organs of C. roscoffensis and of C. paradoxa 

 respectively." Keeble suggests the possibihty that the 

 ultimate death of the animal may not be due to nitrogen 

 starvation, but to "an aggravated attack of 'uric acid 

 trouble.' " 



