112 THE BIOLOGY OF THE SEA-SHORE 



snail, unable to remove it, altered its tactics. The shell, 

 remaining pressed to the glass, turned quickly round through 

 a considerable angle, so that the anterior portion of the 

 outer shell edge came to lie on top of the wedged pincer. 

 By slowly moving its shell backwards and forwards the snail 

 sawed at the limb until the crab withdrew, released its hold 

 and fell to the ground. 



A similar result was obtained, though not quite so satis- 

 factorily, with a piece of wood. In searching for an explana- 

 tion of the snail's conduct, Bauer remarked that the edge of 

 the shell where it pressed on the pincer was beset by a row 

 of sharp teeth which, used in the manner above described, 

 would act as a powerful saw. 



In the light of this phenomenon the relations between 

 hermit crabs and snails seemed to Bauer to be less harmless 

 than has been hitherto imagined, and he set himself to 

 examine them afresh. 



If Gasteropod shells inhabited by hermit crabs are 

 examined many are found to be damaged and the fractures 

 have a characteristic appearance. The last turn of the 

 spiral is broken off exactly as if by small tweezers or bone 

 forceps. Other shells are undamaged, and these belong to 

 particular species. Those of Nassa mutabilis, for instance, 

 often show the typical fracture, while those of N. lacepedei 

 are always undamaged. Bauer attributes these fractures 

 to hermit crabs, and considers the reason for the immunity 

 of certain forms is to be found in the thickened edge of their 

 shells, on which even the strongest hermit crab is unable to 

 get a grip. 



Some shells show periodic thickenings or ribs. For 

 instance, Scalaria communis is distinguished by particularly 

 thick ribs of a strengthening nature. Bauer here comments 

 on the curious fact that he has never been able to find, 

 either in museums or elsewhere, a shell in which the last 

 rib was not exactly at the shell edge. This was the case 

 in spite of the fact that shells, apart from the museum 

 specimens, were collected at all seasons. The portions 

 between the ribs must therefore be formed extraordinarily 



