CKABS FORMING SDCH GALLS. 219 



the coral, it may be concluded with certainty that the crab 



moves about very little in the cavity, for otherwise we should not 



find the very distinct scars which are evidently produced by 



continual scratching in one spot. Since, in all the crabs of this 



group, the current of water for breathing enters the body close to 



the mouth, and passes out again at the hinder margin of the 



branchial cavity, the stream passing through the gall must 



always flow in one and the same direction. The results are 



easily recognisable in the half or wholly 



closed gall. The two excrescences on the 



coral grow together quickest in those spots 



which are least exposed to the current 



through the gall ; there also they first come 



into contact, till at length only two fissures, 



more or less wide, are left, which plainly ^ 



show, by their position opposite to each 



other, thiit it is through them that the cur- ^'"^r^-iiioj^^rT^l^kt 



rent for respiration passes :■ one fissure serves "> "^^ "P™"^ deveiop- 



. . „ , , 1 ment of the com! : b, 



for the influx, the other for the exit, of the the gau with a cavity 



, mi 1 Tj • 1 — here laid open— in 



water, ihese two slits remain open so long whichaorab was en- 



as the crab is alive ; no living crab is ever ° °'^ ' 



found in a closed gall, and they are for the most part perfectly 



empty. 



It is impossible not to conclude from this state of things 

 that the fissures are kept open by the force of the current 

 flowing through them ; but still this can only occur when the 

 force of the current is exactly commensurate with the strength 

 working in antagonism to it, which is exerted by the growing 

 polyps. These are constantly tending, as is shown by the 

 difierent stages of the gall, to reduce the space between the two 

 sides of it ; at first this may be quite easy, but as the force of 

 the current is gradually increased by the diminution of the 

 fissure, at last a stats of equilibrium must be reached in which 

 the forces neutralise each other. Thus, though in the first in- 

 stance the coral was able to continue its growth unhindered, 

 after the manner of its species, it ere long found an obstacle, 

 which it was unable to contend with, in the current produced 

 by the crab. Hence we are justified in supposing that similar 



