Hapalocarcinus, the Gall-forming Crab, etc. 



65 



growth of the polyps around it continues, but the mere mechanical effect 

 of the presence of the crab causes a circular pit to be left, whose depth 

 gives the amount of growth of the colony since the crab first settled 

 down. This pit is almost uniform in diameter, since the growth of 

 the crab is in length and hardly at all in breadth; and it is circular in 

 section, this being the shape of the original theca, to which Crypto- 

 chirus speedily adapts itself. The depth, size, and shape of the house 

 are thus primarily dependent on the polyp and its manner of growth. 

 These characters are not determined by the crab in anything like the 

 manner in which Hapalocarcinus affects those of the gall. Moreover, 

 the shape of the pit is responsible for the form of its inhabitant and not 

 vice versa. Thus, to occupy the circular pit the cephalothorax of Cryp- 

 tochirus has become uniformly cylindrical; the soft abdomen easily 

 adapts itself to the same shape. In order that the creature may raise 

 itself easily and quickly in its long tube and retire to the bottom with 



FIG. 17. Cryptochirus coralliodytes. Third maxilliped. X 50. Compare with that of Hapalocar- 

 cinus in text-figure 5 C. The surface of the ischiopodite is papillated. 



FIG. 18. X 25. A, second pair of thoracic legs, to show the great development of the flat- 

 tened meropodite. B, third pair of thoracic legs. 



equal speed, the four posterior pairs of walking legs are very strong 

 and furnished with very sharp claws to obtain a purchase on the 

 smooth walls of the pit. They are also flattened from side to side, so 

 as to work more easily in the confined space. 



The carapace of the crab is humpbacked, its anterior surface form- 

 ing a thickened disc placed obliquely to the rest. Such a view as that 

 in figure 19 A, shows the mouth of the pit largely filled up by this 

 anterior part of the cephalothorax, while to the sides and in front the 

 chelse and especially the second pair of walking legs help to close the 

 entrance and also to maintain the crab in its position at the mouth of 

 the hole. So here, as in all tubicolous animals, a sort of operculum is 

 formed which closes the mouth of the tube and protects the soft pos- 

 terior parts from injury. 



