40 Papers from the Department of Marine Biology. 
diverging current issuing from the aperture which will repress growth 
on the inner edge of the branch though not on the outer. As the 
outer edges become built up the divergent currents will become 
circular eddies and the growth of the wall of chamber B will follow 
their outside line. The two opposed walls meet naturally at the inter- 
section of their curves, but that part of the current which flows directly 
upwards prevents their entire concrescence. But after the crab has 
migrated into chamber B, partial fusion does occur, so that what was 
at first a wide slit is converted into a series of small circular holes occur- 
ring laterally as well as above, and all of equal size and, presumably, 
importance. This partial closure of the slit is brought about by the 
fusion of the small processes on each side with those of the other. 
These processes are as a matter of fact the twigs springing from the 
modified branches. The effect of the current is not to prevent branch- 
ing but to increase laterally the growth of the branch. The border of 
Fic. 2.—Old established galls in a colony 
of Pocillopora cespitosa. 
The black spots (or line) represent the res- 
piratory apertures (or fissure), the 
dotted circles within the shape of the 
cavity of the gall. aandb are old 
galls, as is shown by the well-estab- 
lished systems of branches which 
spring from their surface. In the 
case of b one of the daughter branch- 
es, which must have started to grow 
after the formation of b, has itself 
been modified to forma gall, which is 
nearly closed. Very possibly, then, b 
and c represent successive generations 
of Hapalocarcinus and afford some 
indication of the ratio of growth as 
between the coral colony and its com- 
mensal. X3. 
the branch is always lobate, each lobe indicating a twig which is not 
allowed to individualise, owing to the continued growth of the branch. 
On the outer side of the gall well-formed twigs are common. In fact, 
one may say that in theformation of gallsthe branching is masked by the 
immediate union of the secondary branches to form a continuous wall. 
A certain number of secondary changes occur after the formation of 
the gall. The polyps inside the gall do not seem to be greatly affected 
by their life within a closed dark space (fig. 4) 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 seriously upon the space in chamber 
B and partially fill up the disused chamber A. This secondary growth 
restricts and eventually stops the lateral movement of the female crab, 
which at that time will have obtained an enormous bulk. The forma- 
tion of the gall does not put an end to the further development of the 
