AN ANALYSIS OF SYMBIONT REACTIONS 93 



and there develops a permanent morphological variation in 

 the host, this would constitute a new species, or, at least, 

 a new variety. The determination of the progenitor of the 

 symbiotic variation might then be purely a matter of chance. 

 A physiological response in the protozoan host would be 

 even more difficult to determine specifically. On account 

 of these obvious difficulties, our analysis must be hmited, 

 more or less, to the multicellular organisms, in which 

 morphologic variations can not only be traced in ontoge- 

 netic development, but may also be recognized in a tissue. 



The simplest type of animal that exhibits a definite mor- 

 phologic response to the presence of a microsymbiont is the 

 Coelenterata. In the Portuguese man-of-war, the cells in 

 the canals of the so-called liver that contain algal sym- 

 bionts are different in appearance from the remaining cells. 

 So, also, in Adamsia diaphana, a hexacoral, the cells of the 

 septa that contain the symbionts are morphologically dis- 

 tinct from the remaining septal cells. It might be said 

 that the morphologic variation in the cells preceded the 

 invasion of the symbionts, and that this imphes a physio- 

 logic variation which produced prototactic conditions that 

 attracted the algae. Such an interpretation, however, 

 would seem to place the "effect before the cause." The 

 special cells are not present before algae make their ap- 

 pearance. A comparison with related, but non-symbiotic 

 species, reveals the fact that the special cells are not pres- 

 ent. Buchner has shown that the algal symbionts are 

 carried in special cells in a large number of animals. These 

 special cells develop in conjunction with the algal sym- 

 bionts, and hence were called "mycetocytes." 



Morphologic reactions in the relative symbiont are spe- 

 cifically shown in the turbellarians, Convoluta roscoffensis 

 and C. paradoxa. In other turbellarian worms, an excre- 

 tory system elaborates and functions, while in the above- 

 named species, the excretory system fails to develop. The 



