THE GONOPHORES OF THE HYDROCORALLIN^. 399 



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 

 thus spread the sexual products farther — was increased by 

 natural selection. Similarly the rim of the gonophore cup 

 was produced until it assumed the size and shape of a medusa. 



The whole of this hypothesis of the origin of the medusse 

 rests upon the supposition that the sexual cells when they 

 reach a certain size set up a local irritation or stimulus, causing 

 a cup-shaped growth of the coenosarc in its immediate neigh- 

 bourhood. 



Is it reasonable to suppose, in the first place, that the gonads 

 when they reach a certain size do produce a local stimulus or 

 irritation? In young immature stocks there is no trace of 

 ampullae or other receptacles in the coenosteum of sufficient 

 capacity for the mature gonads. Nor are there found in stocks 

 that are bearing but few sexual organs any empty cavities in 

 the coenosteum. It is almost certain, then, that the gonads, 

 when they reach a certain size, cause a stimulus to certain cells 

 to secrete an acid (?) which dissolves the lime of the coenosteum 

 jind causes an ampulla to be formed. There can be no doubt, 

 then, that the sexual cells do cause one kind of stimulus to the 

 tissues. 



But is a local irritation or stimulus likely to cause any such 

 modification as circumferential folding of the canals in its 

 neighbourhood ? 



The only directioii in which we can look for an answer to 

 this question is to the effects caused by the irritation of foreign 

 substances and parasites. The Hydrocorallines, like most of 

 the corals, are subject to the attacks of many kinds of parasites. 

 Worms, molluscs, barnacles, and other forms may be seen in 

 every specimen that is examined. 



When the colony is attacked by such a form as Tetraclita, 

 for example, the coenosarc at the immediate spot on which the 

 parasite settles is killed, but this does not cause an atrophy of 

 the surrounding canal system. On the contrary, a pronounced 

 hypertrophy of the canal system immediately surrounding the 



