220 



THE IXFLUE^X'E OF IXANIilATB SUKROUNDINGS. 



hindrances, wliioli might be opposed to the growth, in any 

 spot, of much larger masses of coral, would check or modify 

 their growth in the same way. 



I may for the present postpone the application of the prin- 

 ciple thus ai-rived at ; but it will repay our trouble if we direct 

 our attention to some other phenomena observable in these 

 same galls. The walls of the leaf-shaped excrescences that 



..^^"?^^-%/-' ^ 



Fig. %7 .—Sidpi opoi (I pnlmrtla iMth a pall which is haidly visible from ontside, bnt 

 shows a distinct fismrc dividing the two halves of the closed gall. 



form them bear polyps not only on the outer surface, but on the 

 inner surface as well ; this is proved by the fact that both are 

 closely covered by little depressions, which, from their structure, 

 can only have been formed by polyps. Now, as the polyps 

 situated in the cavity were just as much exposed to the effects 

 , of the current as those on the margin of the fissure, they must 

 show the traces of this influence, if indeed any such influence has 

 been exercised during the growth of the gall. This is, in fact. 



