218 THE INFLUENCE OF INANIMATE SUREOUNDINGS. 



surrounded, and thus enclosed, in the gall which gradually 

 forms. 



It is not difficult to infer what the whole process must be 

 from the different stages observable on one single mass of coral. 

 A diseased excrescence is first produced by the young crab 

 establishing itself between two branches, and the twig thus 

 originating takes various forms according to the character of 

 the species of coral. This is very conspicuous in the different 

 specimens lying before me. In the Seriatopora, both the twigs 

 are leaf-shaped and beset with more or less numerous offshoots 

 terminating ia sharp spines ; in the more solid Pocillopora the 



Fig. 65. — Sidei'opora kystrix, witb galls inhabited by Hapalocardnns mifsupialis. a a a, 

 young galls still half open ; 6, an older one, closed, in which a close inspection may 

 detect two opposite fissures. 



twigs also have spines, but they are more massive ; finally, in 

 Sideropora, spines are wholly absent, and the two twigs between 

 which the crab lives are altogether more massive. In the first 

 instance the two lenf-like twigs are of course far apart, so that 

 the crab could easily get in and out ; but as it does not do this 

 it is soon so surrounded by the growing together of the twigs, 

 that it must remain a prisoner. The creature requires a con- 

 stant and rapid renewal of the water in the gall in which it 

 lives, for the purpose of respiration; at first the water finds 

 a free passage on all sides, but when the two twigs have 

 bent over towards each other, the space through which it can 

 find entrance and exit must grow narrower and narrower. 

 Moreover, from the structure exhibited by galls broken off from 



