HALF AN HOUR WITH CEUSTACEA. 241 



habitation, and almost every one of the latter contains 

 one of these little Pea-crabs inside. It is owing to 

 their living in mussel shells the idea got abroad that 

 it was these crabs which caused the peculiar illness 

 called "musselling," which frequently attacks people 

 who have eaten heartily of them. All we need say is 

 that the notion is completely erroneous. What 

 " musseling " is caused by is not perfectly understood, 

 but there seems reason to believe it is due to eating 

 them when they are in a state of decomposition. The 

 sexes of the Pea-crab differ so remarkably that they 

 have long been taken for different and distinct 

 species. The " Nut-crabs " (Ebalid) require a sharp 

 eye to detect them, for the largest is little over half 

 an inch long. Their shell is lozenge-shaped, with 

 rounded angles. All the claws are, with the excep- 

 tion of the foreclaws, usually tucked under the cara- 

 pace, so as hardly to be visible. There are several 

 species of this genus, not uncommon especially on 

 our southern coasts. Like many others of the crabs, 

 they sham death when caught, and are also so modified 

 by "mimicry," that one is apt to take their dull 

 white carapaces for quartz pebbles. They usually 

 burrow in gravel, and are nocturnal in their habits. 

 The " Angled-crab " (Oonoplax angulatus) well 

 deserves its name, as it is certainly the most angular- 

 shaped of any of our British crabs. The hind limbs 

 are flattened, and may be used for swimming, espe- 

 cially as it frequents deep water. At Dublin this 

 species goes by the name of "Coffin-crab." The 



