THE CRAYFISH AND OTHER CRUSTACEA 



93 



seaweed, sponges, hydroids, etc., so that it is difficult to 

 detect the animal in its natural surroundings. It is a 

 curious fact that these growths may be planted by the 

 crabs themselves. A spider crab placed in an aquarium 

 with bits of seaweed will snip off pieces with the pincers, 

 reach back and place them among the short, hooked spines 

 and hairs on its carapace where they become attached and 

 grow. A crab may deck itself out with bits of paper 

 in a similar manner. It has been found by experiment 

 that this wonderful instinct does not depend upon the 

 brain, as a crab will proceed to disguise 

 itself in the same way after the brain is 

 entirely destroyed. 



The foregoing crustaceans, however 

 much they differ in external appear- 

 ance, agree in having many character- 

 istics common by which they are grouped 

 in one order, the Decapoda. The term 

 has reference to the possession of ten 

 legs which is a general feature of the 

 group. The Decapoda also have a 

 carapace, stalked eyes and gills on the 

 cephalothorax. 



Another order standing somewhat lower in the scale is 

 the Isopoda. In this group the typical number of legs is 

 fourteen; there is no carapace, the eyes are not stalked but 

 sessile, and the gills are formed by modifications of the 

 appendages of the abdomen. There are many marine 

 isopods, some of which are parasites of fishes, while some 

 very degenerate forms prey upon other crustaceans. 

 One species, Limnoria lignorum, is a serious nuisance to 

 man, as it bores into the piles of wharves and so riddles 

 them with its burrows that they soon become useless. A 

 few isopods, Asellus and its allies, occur in fresh water, 



FIG. 78. A sow 

 bug, Porcellio, en- 

 larged. (After Es- 

 sig.) 



