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 



FIG. 2. Old established galls in a colony 

 of Pocillopora ccespitosa. 



The black spots (or line) represent the res- 

 piratory apertures (or fissure), the 

 dotted circles within the shape of the 

 cavity of the gall, a and b 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 form a 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. Xf. 



- -C 



-b 



a- 



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 the formation of galls the 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 

 ccenenchyme 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 



