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. 



Caiman says that in the galls examined by him, the thecse 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 Cai- 

 man's observations were made on the former; and my own agree with 

 them. Inside the gall the thecse are shallower, sometimes prolonged 

 towards the border of the gall, and differ considerably in size. The 

 septa are less distinct. There is a larger amount of ccenenchyme inter- 

 vening between the thecse. 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." 



