CRABS FORMING SBCH GALLS. 219 



the coiul, it may be concluded witTi 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 sci-atching 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 t 



closed gall. The two excrescences on the "^J'X 



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, that it is through them that the cur- ^'"i^AM^^rT^m'^l 

 rent for respiration passes ; one fissure serves «■ *= normal deveiop- 



^ ^ ' ment of the coral : 6, 



for the influx, the other foi- the exit, of the the gaii with a cavity 



, . —here laid open-in 



water. These two slits remain open so long which a crab 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 

 different 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 state 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 



