48 THE STORY OF LIFE IN THE SEAS. 



Another great class of animals which has many 

 representative forms among the rocks is the group 

 of Crustaceans. The Lobster, the Prawn and the 

 Crab are all familiar examples of this class. 

 They may be found by searching rock pools at 

 low water, or can be captured in basket-work 

 traps in places beyond low-water mark. When 

 undisturbed they crawl slowly over the rocks 

 and weeds by their long jointed legs, searching 

 for their prey, but when alarmed the Lobster 

 and the Prawn can, by violent flapping move- 

 ments of their tails, dart rapidly backwards 

 through the water, while the Crab beats a 

 hasty retreat sideways into some shelter among 

 the rocks. Like many of the Molluscs, the 

 Crustaceans have a hard covering or shell to 

 protect them from many of the dangers to which 

 soft-bodied animals would be exposed, but a 

 momentary glance at them would be sufficient to 

 satisfy the most inexperienced eye that there are 

 many and important differences in the character 

 of the shells of these two great groups of animals. 

 One important distinction between them, how- 

 ever, might well escape observation, and that is, 

 that whilst in the Mollusc the shell increases 

 gradually in size during the life of the animal, in 

 the Crustaceans it cannot do so. In the Lobsters 

 and Crabs the shell is periodically cast off entirely, 

 and for a day or two at each period the skin of 

 the animal is quite unprotected. A new shell is 

 gradually formed, and this is hardened and thick- 

 ened until it assumes a form similar to that of 

 the one that has been lost, but larger. During 

 the moult the Crustacean usually hides itself in 



