120 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



hydroids or sea-anemones attached to their shells. Both 

 animals doubtless derive some benefit from this association, 

 the crab being protected from its enemies and the hydroid or 

 anemone being moved from place to place where food is more 

 abundant and perhaps gathering some of the bits that are 

 scattered in the water when the crab is feeding. This is 

 another example of commensalism, or symbiosis. 



The sand-bugs, genus Hippa, so numerous on sandy beaches, 

 burrowing rapidly into the loose sands or sometimes swimming 

 about in the tide pools, are an important source of food for 

 some fish. 



Many other interesting forms in this sub-order, some of them 

 of more or less economic importance, might be listed, but the 

 kinds already mentioned serve to show something of the 

 diversity of structure and habits of the group. 



Crabs. The true crabs, belonging to the sub-order Brachy- 

 ura, differ from the crayfish and lobsters in having the body 

 short and broad instead of elongate and rounded. The 

 cephalothorax is often broader than long. The abdomen is 

 relatively small and is bent under the cephalothorax so that but 

 little of it is visible from above. The appendages are usually 

 well developed and similar to those of the crayfish or lobster 

 except that the number of abdominal appendages is reduced 

 to two pairs in the male and four pairs in the female. Most 

 crabs are scavengers, living on dead animal matter. During 

 their development they undergo a remarkable metamorphosis. 

 In some of these stages they are so unlike the adult that they 

 were described as different animals. The names, such as 

 zoea and megalops, given to these supposedly distinct animals, 

 are still retained, but we know now that they refer to different 

 stages in the development of the crab. 



Most of the crabs live in the shallow waters near the shore, 

 but some live in deep water and a few live on land. Their 

 habits are various and there is a corresponding variation in 

 their structure and shape, size and coloring. 



The spider crabs, with their long, slender legs and com- 

 paratively small body, are especially strange looking creatures. 

 Some members of this group living in Japanese waters measure 



