398 SYDNEY J. HICKSON. 



it feeds and maintains itself, is in a morphological sense simply 

 the detached generative organ of the hydrosoma on which it is 

 developed." 



The gonophores of the Hydrocorallinge do not seem at first 

 sight to throw much light upon these questions. If we 

 arbitrarily assume that they are degenerate medusae comparable 

 to the adelocodonic gonophores of the Tubularise and Cam- 

 panulariae, we cannot expect to find in them any evidence to 

 support either the one view or the other. But there is no 

 reason to suppose that they are degenerate medusiform gono- 

 phores. Neither in Millepora, nor in AUopora and Disticho- 

 pora, are there any features in development that suggest rudi- 

 mentary structures of medusae. 



If they are not degenerate structures, then, but gonophores 

 of a primitive type, how can we reconcile the medusa of Mille- 

 pora, which is a metamorphosed polype, with the gonophores 

 of AUopora and Distichopora, which show no trace of polypoid 

 or medusoid structure? 



The explanation I would suggest is briefly as follows : 

 When the ova or sperm-mother cells reach a certain size and 

 are too large to move freely in the canal system, they set up a 

 local stimulus or irritation, which causes a cup-shaped folding 

 of the adjacent canal or polype wall. This cup-shaped fold 

 being of advantage to the sexual cells during their maturation, 

 by aflFording increased facilities for nourishment and by in- 

 creasing the size of the cavity by solution of its walls, has been 

 modified into a definite form in each species by natural selec- 

 tion. When the sexual cells arrive at their maturity the 

 nourishment afl'orded by these cells is no longer necessary, 

 and consequently the stalk of connection with the canals be- 

 comes constricted until the gonophore is set free in the cavity 

 of the ampulla. In the ancestral form of the Millepora a 

 ready access to the exterior was open to the separated gono- 

 phore by way of the dactylopore, and thus the detached gono- 

 phore was able to escape and lead a free-swimming existence. 



It is reasonable to suppose that all the cells of the colony of 

 a Millepora are capable of a certain amount of contractility, 



