148 



CORALS 



organisms called zooxanthellae, the function of which has 

 been discussed in a previous chapter (p. 20). 



The coenosarcal canals are confined to the outermost 

 layer of the corallum. Down to the level of the first tabula 

 (Can. I in Fig. 67) they are alive and functional ; below that, 

 for a distance represented by two or three tabulae, they are 

 shrivelled and degenerating, and below that again they 

 disappear altogether. 



Thus when a branch of a Millepora preserved in spirit, 

 say half an inch in diameter, is 

 treated with acid and the corallum 

 dissolved away, the whole system 

 of canals and polyps is represented 

 by a film not more than -^J^ of an 

 inch in thickness. 



The colonies of Millepora are 

 richly supplied with nematocysts, 

 and as some of them are powerful 

 enough to pierce the human skin, 

 causing a painful form of nettle- 

 rash, the Millepora is regarded as 

 a stinging coral. The nematocysts 

 are of two kinds : a smaller kind 

 found in the tentacles of the zooids, 

 armed with four sharp spines at 

 the base of the filament, and a 

 larger kind without spines but with 

 a much longer filament which are 

 scattered over the surface of the 

 coenenchym between the zooids (Fig. 69). 



The reproduction of Millepora is of extraordinary interest, 

 because it presents us with the only example that is known 

 of a stony coral that produces free-swimming medusae. The 

 medusae are produced in great numbers, they are of a very 

 simple structure, and when a colony is examined, are found 

 to be of the same sex, either male or female, and at approxi- 

 mately the same stage of development. 



There are many points about this production of medusae 

 in Millepora on which we are still in ignorance. It is not 



Fig. 69. — The nematocysts of 

 Millepora. A, The large kind 

 with the thread discharged. 



B, The same before discharge. 



C, The small kind with thread 

 discharged. D, The same before 

 discharge. x 700 diams. 



