222 THE EVOLUTION OF THE META20A 



EndosyMbiotic Monocellular Algae 



The fact that the so-called zooxanthellae and zoochlorellae 

 which live intracellularly can be frequently found in Cnidaria 

 as well as in Turbellaria cannot yet alone be considered as a di- 

 rect proof of a close relationship between these two animal 

 groups; in particular it does not help us to distinguish which of 

 the two groups is the parent and w^hich the descendant. 

 Nor is it proved that this appearance can be considered as a 

 pure coincidence only. This situation, however, is altered when 

 we notice that in the Turbellaria the mutual relationship between 

 the animal and the alga stands on a more primitive level than 

 the level which has been reached in this connection in Cni- 

 daria. So far the physiological circumstances of this symbiosis 

 have not been clarified (the extent of the mutual dependence as 

 regards breathing and feeding); we w'ill take into considera- 

 tions those conditions only that can be observed during 

 the transmission of symbionts frogn one generation to the 

 other. 



We can disregard the circumstance that there are at least 

 two types of symbiotic algae, the green coloured zoochlorellae 

 (this is, as a matter of fact, not a taxonomic idea), and the 

 zooxanthellae whose colour is yellow^ to brow^n (they are actu- 

 ally Dinoflagellata, according to Chatton). The situation is 

 usually such that zooxanthellae prevail in those animal partners 

 that live in the sea, while zoochlorellae are more numerous in 

 those partners that live in fresh w^ater. It should also be mention- 

 ed here that such endosymbioses occur as early as in Protozoa 

 (especially in Radiolaria and in Euciliata). As for the Turbel- 

 laria, we can read in the manual written by Hyman (11:143), 

 "Most acoela and some rhabdocoela and alleocoela harbour 

 symbiotic chlorellae and xanthellae;" these live mainly in the 

 mesenchymal cells under the skin muscle tube. It is important 

 in this connection that these algae do not pass over into the 

 egg cells so that the young animals which have just developed 

 from an egg do not possess symbionts and they must be later 



