38 Papers from the Department of Marine Biology. 
Seriatopora, on the other hand, shows less power of response to 
external conditions. While it flourishes most luxuriously on the mid 
regions of thegreat reef flat at Murray Island, it rarely approaches within 
a hundred yards of the edge, as does Pocillopora. Galls are by no 
means so common on its colonies, for they represent a much greater 
interference with the normal type of branching. The introductory 
remarks, then, which follow are confined to Pocillopora, the abundance 
of the material of which gave exceptional facility for study. 
In the first place it may be stated that the galls themselves vary 
greatly in development and that there is a general correspondence 
between the stage of development and the size of the crab it contains. 
In all cases the gall appears to be formed and inhabited by a solitary 
female individual. Those in the younger “‘open”’ galls, where the two 
constituent branches have not yet approached each other and fused, 
are immature; those in the older ‘‘closed”’ galls are mature, and often 
the abdominal appendages are laden with developing eggs. 
THE GALLS IN POCILLOPORA CASPITOSA. 
The young female crab probably commences its sedentary life by 
settling down in the notch at the apex of a recently divided branchlet. 
She is at this period a small flat creature, little more than a millimetre 
in carapace length. Her position at the growing point allows her to 
control the future development of the branch. To estimate the extent 
of her influence a more exact description of the Pocillopora colony must 
be given. Though the method of branching is certainly dichotomous, 
it is very irregular. If we first of all take the still-water type of colony, 
here the main branches are comparatively slender, not more than 5 to 
7 mm. in diameter. Dichotomy is well shown in some of the main 
divisions. It is always exhibited in the growth of the lateral and 
terminal twigs, which closely beset the main branches. These twigs 
are usually 3 to 4 millimetres long and not more than half a millimetre 
in diameter. In the course of their growth they are at first quite cylin- 
drical; then they become palmate, and finally, as growth is checked in the 
middle, division takes place and two short equal divisions are formed. 
The space between these when they assume their palmate shape is 
sufficiently large to accommodate and partially protect so small a 
creature as the young Hapalocarcinus. 
When the colony lives in troubled waters its form changes consider- 
ably. The main branches are much shorter and stouter; the twigs 
which cover their surface are more numerous and more closely set. 
They do not exhibit dichotomy, but always remain cylindrical and do 
not broaden out apically, so that they do not promise good shelter to 
a householder whose size increases so rapidly as does that of Hapalo- 
carcinus. 
When the crab settles down the effect of its presence is very soon 
felt. The two branches, instead of diverging at an angle, remain at 
