STAGES IN THE LIFE HISTORY OF A SYMBIOTIC ZOOXANTHELLA IN PELLETS EXTRUDED BY ITS HOST AIPTASIA TAGETES (DUCH. AND MICH.) (COELENTERATA, ANTHOZOA).

1. Various morphological stages in the life history of the zooxanthella symbiotic with Aiptasia tagetes have been identified in pellets extruded by the actinian host. One of the stages is a motile zoospore. 2. This zoospore resembles that observed by Kawaguti (1944) and differs from those identified in axenic cultures of G. microadriaticum in containing a large accumulation body. Suggestions are made as to the significance of this difference. 3. It is very likely that the zoospore is an infective stage in the life history of the zooxanthella. 4. There is some evidence to suggest that indirect infection of hosts by zooxanthellae is also possible.