PREVIOUS INTERPRETATIONS OF CNIDARIA 223 



"infected" by food which contains these algae, e.g. by the egg 

 shell whose surface is covered with these algae. 



It is characteristic that no case of a symbiosis with these algae 

 is known in Polycladida and that correspondingly zooxanthel- 

 lae do not occur in the completely transparent pelagic Cteno- 

 phora which derive from Goette-Miiller's larvae that have 

 adopted neoteny— even if such a symbiosis could be expected. 



In TurbeUaria the majority of cases of endosymbiosis can 

 be found in Acoela and in other primitive groups. It is not 

 impossible that in Acoela the absence of nephridia could 

 be attributed at least partly as being due to this endosym- 

 biosis since the algae which Hve in the cytoplasm use up the 

 products of excretion of their partners. The physiological 

 relationship with these algae can be very complex in numerous 

 species of Acoela (e.g. in species belonging to the genus Con- 

 voluta) . Yet in spite of this we find that the continuity of this 

 symbiosis has not been secured in the most reliable way, i.e. 

 by means ot a transition of algae into the egg cells. It could 

 perhaps be said that as early as in Acoela the eggs are better 

 isolated against their surroundings in the body of the partner. 

 The possibility, however, cannot be excluded that in those 

 TurbeUaria which regularly possess symbiotic algae, cases 

 will be found w^here symbionts are directly transferred by 

 means of egg cells. This could be the case in Acoela with a pri- 

 mitive sexual apparatus. Von Graff (1908, 1943) admits the 

 possibility that in the species Volychoertis caudatus Mark, the 

 algae are present even in the cleaving egg. 



It is well known that in Cnidaria a symbiosis with mono- 

 cellular algae can be frequently found. We can observe such 

 cases in all the three subdivisions, and most frequently in 

 Anthozoa, among others in Actiniae and in Madreporaria. This 

 symbiosis occurs so regularly in the cUff-building corals, 

 whose most profuse growth takes place in the well-lit upper 

 zone of water, that earlier it was quite generally believed that 

 these corals lived and grew mainly or even exclusively at the 

 expense of their endosymbionts. This endosymbiosis, however, 



