STRUCTURE AND CLASSIFICATION 21 



physiology of corals cannot be fully estimated at present, as 

 there are many points in the relationship between the 

 svmbionts that are in need of further investigation ; but 

 some idea of the importance of the association may be 

 conveyed by a brief statement of the facts that are known 

 about a single example — the Millepora coral. 



Millepora is a large massive or branching coral (see p. 145) 

 which is found in shallow water all over the tropical world, 

 and wherever it is found the superficial plexus of canals is 

 always crowded with zooxanthellae. 



No specimen has yet been examined either in the East 

 Indies or the West Indies in which the zooxanthellae do 

 not occur in abundance. It is not a case, therefore, of an 

 infection confined to certain specimens or certain localities. 



Moreover, it has been shown, in the case of Millepora, 

 that there is no stage in the life-history of the coral in 

 which it is free from this infection. The young egg cells in 

 the ovary, long before they reach full size and maturity, are 

 invaded by zooxanthellae from the surrounding tissues, and 

 thus, when the egg is fertilised and develops into a larva, 

 it is already provided with a full equipment of these sym- 

 biotic cells. ^ 



As no specimens of Millepora have yet been found without 

 the zooxanthellae, we cannot tell if this coral can manage 

 to exist without them, nor can we assert without experi- 

 mental proof that the association is of any benefit to it. 



But similar cases of s3^mbiosis are known in other animals, 

 and in one of these — the symbiosis of the little flat worm 

 Convoluta with zoochlorellae — it has been shown experi- 

 mentally that the Convoluta is dependent on substances 

 formed by the zoochlorellae for at least an essential part of 

 its nutrition. 2 



If, as seems highly probable, there is the same kind of 

 relation between Millepora and its zooxanthellae as there is 

 between Convoluta and its zoochlorellae, the holozoic method 

 of nutrition of the coral is supplemented by the holophytic 

 action of the chlorophyll-bearing zooxanthellae. 



^ J. Mangan, Quart. Joitrn. Micr. Sci. ^2, 1909. 

 ^ Gamble and Keeble, Quart. Journ. Micr. Sci. 51, 1907. 



