THE ARTHROPODS 119 



which they poke into the face of any intruder. While 

 the anemones or hydroids are made to do valiant service 



FIG. G6. Hermit-crab (Pagurus bernhardus) in snail shell covered with Hydraclinia. 



with their nettle-cells, they also enjoy the advantages of 

 a large food-supply which is attendant upon the free ride. 



114. The crabs. The most highly developed Crustacea 

 are the crabs or short-tailed decapods which abound between 

 tide-marks alongshore, and in diminishing numbers extend 

 to great depths. The cephalothorax is usually relatively 

 wide, often wider than long, and the greatly reduced abdo- 

 men is folded against the under side of the thorax. Corre- 

 lated with the small size of the abdomen, the appendages 

 of that region disappear more or less, but the remaining 

 appendages are similar to those of the crayfish or lobsters. 

 All these different parts, however, are variously modified in 

 each species to fit it for its own peculiar mode of life. In 

 some forms, such as the common cancer-crab (Fig. G7), the 

 legs are comparatively thick-set and possessed of great 

 strength, enabling them to defend themselves against most 

 enemies. On the other hand, there are the spider-crabs 

 with small bodies and relatively long legs, withal weak, and 



