CRABS FORMING PITS. 221 



quite unmistakably the ca.se. Xct one of the cups is normal 

 in structure ; the depression, which in the external polyps is 

 very deep, is here no more than a shallow pit, and the septa (or 

 party walls) of the cup are very slightly developed. Hence it 

 follows, 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 cnps, 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. 



We see from this that the current caused by the crab is 

 sufficient not merely to force the diseased excrescence on the 

 coral to take a particular, direction, but also to check the growth 

 of the individual polyps quite as considerably, and to divert 

 them from their normal growth. 



The influence of the respiratory current of crabs of the 

 genus Cryptochirus, which live only in the more massive forms 

 of coral, appears to be exerted in quite a different way. I 

 found them in the Philippine Archipelago in cavities in 

 Goniastrcea, Bournoni, in an undetermined true Astrcea, which 

 was unfortunately lest, also in an undescribed Trachyphyllia ; 

 finally I received a new form through A. Agassiz from the 

 West Indian seas, which may perhaps form a distinct genus, 

 though it is very nearly allied to the first. It also lives in a 

 Trachyphyllia. 103 The reader will see that they all belong to the 

 massive corals, and, in correspondence with this circumstance, 

 the cavities in which these crabs live are totally unlike those 

 in which Hapalocarcinus is found. Here there are no galls, 

 but merely cylindrical or funnel-shaped hollows, which are 

 never closed during the lifetime of the crab, so that it cer- 

 tainly would be able to quit its position. Nevertheless, it as 

 certainly does not do so ; but the species I observed living 

 thrust the forepart of their bodies very far out of their pecu- 

 liar ' cave-dwellings/ so that only their pouches, i.e. the hind 

 part of the body, remained within. The cavity itself exhibits 

 some remarkable peculiarities. The bottom of it, on which the 



