ASSOCIATIONS 613 



porarians, but in cold northern seas are confined to a few actiniarians 

 such as Anemonia sulcata. 



The zooxanthellae of coelenterates are highly specialized for life within 

 the animal, are localized in the endoderm of corals and other forms and 

 are transmitted from generation to generation by way of the egg and planula 

 larva. A free-living stage appears to be lacking and they are not found free 

 in the sea. The association is thus essential to the zooxanthellae which can 

 live only within corals or similar animals. Under laboratory conditions 

 motile flagellates have been cultured from coelenterate zooxanthellae, 

 however (64a). Among those species of corals and anemones which 

 normally harbour zooxanthellae, individuals which live in dark places 

 can do without their algal symbionts and obtain all their nourishment 

 holozoically. There are some coelenterates (Xeniidae) which have lost 



Fig. 14.19. Section through a Ventral Mesenterial Filament 



of the Alcyonarian Sclerophytum capitate 



Zooxanthellae are numerous. (From Pratt, 1905.) 



the capacity of capturing their own food, and are dependent upon their 

 imprisoned algae for foodstuffs (33). 



In hydrozoan corals (Millepora), alcyonarians and madreporarians, 

 the zooxanthellae occur only in the endoderm (Fig. 14.19) and are most 

 abundant in those regions, such as disc and tentacles, which are exposed 

 more fully to the light. In the Scyphozoa, however, the zooxanthellae are 

 situated in the mesogloea. In all cases the unicellular algae are intracellular, 

 and are held in amoeboid wandering cells (96, 108). 



Turbellaria. The best-known example of symbiosis in marine turbellar- 

 ians is afforded by Convoluta (Acoela) (Figs. 14.20 and 14.21). C. convoluta 

 (= paradoxa) contains zooxanthellae which occur in large numbers 

 just below the epidermis. This animal retains its alimentary canal and 

 continues to feed normally, but it is also dependent upon lipoids manu- 

 factured by its algal symbionts and fails to develop in their absence. When 

 starved it eventually digests the zooxanthellae, but can be infected again 

 from the outside. The eggs are free of algae, and the larvae, colourless 

 at first, are infected by free-living algal cells (55). 



In C. roscoffensis the symbiotic association is still more intimate. At 

 first the animal feeds holozoically, but when mature it depends on the 



