94 SYMBIONTICISM AND THE ORIGIN OF SPECIES 



absence of this system in these worms is associated with 

 the presence of algal symbionts. Again, it might be argued 

 that this failure in development of an excretory system 

 produced prototactic properties in the worms that are 

 specific for certain algae, and that this circumstance is the 

 crucial factor in the development of the symbiotic rela- 

 tionships. This explanation would not supply a cause for 

 the lack of development. Since excretory systems are 

 present in closely related, but non-symbiotic species, and 

 in the absence of any other evident factor, it appears 

 that the symbiotic algae may be responsible for the absence 

 of the excretory system in Convoluta roscoffensis and C. 

 paradoxa. It is not possible for us to explain the nature 

 of the mechanism whereby the presence of the algae brings 

 about the permanent agenesis of an entire organ system. 

 We can understand how the loss of activity of an organ 

 may result in complete atrophy, but we do not know how 

 such a deficiency can be so impressed upon heredity that 

 the ''defect" shall be permanent in succeeding generations. 

 The presence of a foreign microorganism, suggests the 

 possibility that the microsymbiont in some way influences 

 the germplasm of the host. The absence of the symbionts 

 in the eggs of the host, also complicates the situation. True 

 the symbionts are present in the egg capsule which contains 

 the elements of food necessary for the symbiont, but the 

 symbionts do not enter the developing worm until it has 

 reached a certain stage in its life cycle. We must assume 

 that if the microsymbiont impresses a new characteristic, 

 or perhaps, better stated, destroys a potential characteristic 

 in the germ plasm, this must be accomplished while the 

 microsymbionts are present in the general or somatic tis- 

 sues of the host. 



The loss of the property of food-ingestion in Convoluta 

 roscoffensis, indicates a definite and fixed physiologic re- 

 sponse to the presence of the algal symbionts. This 



