THE DECAPOD CRUSTACEA. 



197 



interest to those who care about the problems of evolution. 

 Not only are they well adapted for their function of bearing 

 the spider-crab's "forest of Dunsinane," but the crab itself 

 actually attaches weeds and zoophytes to them. When the 

 hairs are removed the carapace will be seen to be covered 

 with the numerous tubercles so characteristic of the spider- 

 crabs in general. It is dull in tint, inclining towards red 

 on the upper surface, young and small specimens being 

 often very distinctly red in colour. 



The general points named above having been made out, 

 we may proceed to consider the special characteristics of 

 structure. 



Fio. 55.Hyas araneus, the common spider-crab. The coating of 

 hairs is only indicated. 



The carapace is broad, elongated, and triangular, only 

 slightly arched, and prolonged anteriorly into a bifid 

 rostrum, whose converging halves are flattened above and 

 deeply hollowed beneath. Immediately behind the orbit 

 there is a very characteristic spear-shaped process. The 

 abdomen is seven-jointed in both sexes. In large speci- 

 mens the carapace may have a length of over three inches, 

 and a breadth of over two. In such a specimen the legs 

 would be over five inches long. The specimens ordinarily 

 found on the rocks are, however, likely to be smaller than 

 this. 



