614 



THE BIOLOGY OF MARINE ANIMALS 



excess of food manufactured by the algae. Finally, it consumes its algae, 

 reproduces and perishes. Again, in this species the larvae when hatched 

 are colourless. Free-living algae are attracted by chemotaxis to the egg 

 case, and infect the young animals when they emerge. In this association 

 we can regard the algae as really constituting an organ of the animal : the 

 latter, in fact, is living parasitically on its contained algae (56). 



Mollusca. Among marine forms, algal symbiosis has been described in 

 several species of opisthobranch gastropods, and in a few lamellibranchs. 

 In the nudibranch Aeolidiella glauca, zooxanthellae occur in cells of the 



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Fig. 14.20. (a) Convoluta convoluta and (b) C. roscoffensis, Two Species of 



ACOELOUS TURBELLARIANS HARBOURING SYMBIOTIC ALGAE (X 27). 



(From Keeble, 1912.) 



liver tubules which extend into the cerata (finger-like processes arising 

 from the dorsal surface of the body). These animals prey upon the anemone 

 Heliactis bellis, from which they acquire their zooxanthellae (37). Other 

 sea-slugs which feed upon anemones and contain zooxanthellae probably 

 obtain them from the same source. In Tridachia crispata, which is a 

 herbivore, the zooxanthellae lie in connective tissue and are localized 

 in a band lying near the margin of the highly flattened body. The pelagic 

 form Phyllirrhoe also harbours zooxanthellae in liver diverticula. Zooch- 

 lorellae in Placobranchus occur in folds of the branchial cavity, and in the 

 liver where they may be undergoing digestion (12, 52, 111). 



Symbiosis with algae is highly developed among the Tridacnidae 

 occurring on coral reefs of the Indo-Pacific region (Fig. 14.22). Two 



