Hapalocarcinus, the Gall-forming Crab, etc. 45 
with some degree of certainty, that the polyps on the inner surface were not 
able wholly to overcome the resistance of the current passing over them. This 
direct action of the stream is unmistakable in many of the cups, where the 
polyps were exposed to the greatest force of the current produced by the crab; 
for they are placed obliquely on the fissure and directed outwards, as they 
must have grown, supposing them unable to grow against the stream.” 
The current, then, Semper thinks, modifies the direction of growth of 
the polyps. 
Calman says that in the galls examined by him, the thece of the inner 
surface are, ‘‘as Semper states, small and shallow and have the septa 
only feebly developed. They are also in some parts distorted and 
drawn out to an elliptical outline, but this distortion does not appear 
to be definitely related to the marginal aperture of the gall as described 
by Semper, who attributes it to the action of the current of water caused 
by the crab.”’ This qualifies the force of Semper’s conclusion. In Pocil- 
lopora there is certainly a remarkable difference between the polyps 
on the inside and outside of the galls (fig. 4). This is seen on examina- 
tion of both the hard skeleton and the soft parts. Semper’s and Cal- 
man’s observations were made on the former; and my own agree with 
them. Inside the gall the thece are shallower, sometimes prolonged 
towards the border of the gall, and differ considerably in size. The 
septa are less distinct. Thereisalarger amount of coenenchyme inter- 
vening between the thece. A considerable disturbance of the normal 
method of growth, due to the respiratory current of the crab, is clearly 
shown. 
The soft parts of the colony were studied after decalcification in 70 
per cent alcohol with a small amount of nitric acid. Total prepara- 
tions of the superficial tissue were made, staining in borax-carmine and 
mounting in Canada balsam. On the outside of the gall the polyps 
are regular in shape and position; they are nearly all of the same size. 
Internally a large proportion are small, though they usually have the 
total normal number of mesenteries and can not be regarded as juvenile. 
The larger polyps have not the regular shape characteristic of the 
normal forms. The examination of these preparations shows that 
the main effect of the current is direct repression of growth. The 
polyps which are formed are not allowed to attain the normal size. 
The distortion which they undergo is quite a secondary and unimpor- 
tant effect. 
Certain points of structure will now be described which shed some 
light on the biology of Hapalocarcinus. The first is the curious nature 
of the buccal area and the appendages surrounding it. Stimpson in 
the original description mentioned the large buccal area occupying 
nearly the whole breadth of the carapace and the third maxillipeds 
“with the ischium large and dilated within, while the merus is very 
small and slender like the three last joints.” 
