204 SYDNEY J. HICKSON. 



The gonophores of the Hydrocorallinse do not seem at first 

 sight to throw much light upon these questions. If we arbi- 

 trarily assume that they are degenerate medusae comparable 

 to the adelocodonic gonophores of the Tubularise and Cam- 

 panularias, 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 Allopora and Distichopora, 

 are there any features in development that suggest rudimentary 

 structures of medusse. 



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 Allopora 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 affording 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 

 selection. When the sexual cells arrive at their maturity the 

 nourishment afforded by these cells is no longer necessary, and 

 consequently the stalk of connection with the canals becomes 

 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 opened to the separated gonophore by way 

 of the dactylopore, and thus the detached gonophore 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, 

 and that the slight power of contractile movement that the 

 original free gonophore possessed being of advantage to the 

 species— by enabling the gonophore to keep afloat longer and 



