Hapalocarcinus, the Gall-forming Crab, ete. 37 
growth of the colonies can be modified by external agencies accounts for 
the attraction which Pocillopora possesses for H. apalocarcinus. No coral 
shows more variation under different environmental conditions. If 
colonies of P. cespitosa from the outer edge of the reef are compared 
with those which grow in the sheltered inner waters it is difficult to 
believe that the two forms can be embraced within a single species. 
The compact colonies with short thick branches in the former situation 
owe their form to the strong wave action, while the long and slender 
branches which the coral develops when out of reach of the breakers 
are correlated with the absence of strong currents within the reef. 
Fria. 1.—Various stages of gall-formation in Pocillopora cespitosa. 
A. Normal branch of colony. 
B. Tip of growing branch in two stages, showing dichotomous division. 
C-F. Galls in side view to show both the component branches. 
C. First stage of formation of a gall; the dotted line indicates the extent of the chamber 1. 
D. A typical “open” gall: chamber 1 inhabited by crab, chamber 2 in course of formation. In 
this and the following figures E-G the gall has been cut in vertical transverse section 
and ground down by stone. The shading shows the ground surface. 
E. A “closed” gall with chamber 2 completed and occupied by the crab; chamber 1 deserted 
and partially filled up by spongy ccenenchyme. The black spots show the position of 
the thee of the coral. 
F. An older “closed” gall showing increased thickness of walls and restriction of interior space, 
G. Diagram of an “open” gall to show the way in which the respiratory current influences 
the growth of the coral. The arrows represent the expired current, the dotted lines 
the direction the growth of the walls will take. 
H. Diagram of ‘‘closed”’ gall in plane of union of the component branches, showing the rela- 
tion of the inhabitant to the apertures, a,a,a. A complete series like this is found in 
the earlier stages of closed gall, but becomes interrupted later. 
I. View of an “open’’ gall from above, showing the converging laminar branches; their border 
is seen to consist of a series of short processes. One or two twigs have been formed 
on the outer surface. 
K. View of “closed” gall from above, showing apertures as black spots and twigs arising 
from all over the outer surface of the gall. 
All the above drawings are natural size except of B and G, which are slightly enlarged. 
