462 Mr Potts, The gall-forming Crab, Hapalocarcinus. a 
repress growth on the inner edge though not on the outer. As | 
the outer edge was built up these oblique currents would become 
circular eddies and the growth of the wall of chamber B would © 
follow their outside line. The two opposed walls meet naturally 
at the intersection of their curves but that part of the current 
flowing directly upwards, in its escape prevents concrescence. But | 
after the crab has migrated into chamber B, partial fusion does : 
F 
occur so that what was at first a wide slit is converted into a series « 
of small circular holes occurring laterally as well as above, and all 
of equal size and, presumably, importance. There is nothing im | 
my material to support the curious observation of Semper* in 
Transverse sections of Galls to show stages of formation. 
1. An ‘open’ gall, chamber B in course of formation. 2. Diagram of an | 
open gall to show the way in which the respiratory current may influence the ~ 
growth of the coral. The arrows represent the expired current. 3. A ‘closed’ gall, 
chamber B completely formed. Chamber 4 has been almost filled up. 
The crab inhabiting the gall lies in a plane at right angles to the paper and so — 
is represented in side view. The black spots inside B represent the thecae of 
polypes. 
which he describes the concrescence of the edges proceeding “ till 
at length only two fissures,...are left, which plainly show by their 
position opposite to each other that it is through them that the — 
current for respiration passes: one fissure serves for the influx, 
the other for the exit, of the water.” I think that though the © 
crab cannot turn round, it moves about freely in a vertical plane — 
and that all the apertures are used in turn. 
A certain number of secondary changes occur after the 
formation of the gall. The polypes inside the gall do not seem to 
be greatly affected by their life within a closed dark space and 
the thickness of the coenenchyme is added to very distinctly on 
the inside as well as the outside, so much so as to encroach ~ 
* Karl Semper, Animal Life, English Translation, 1881. 
