CRUSTACEANS. 



487 



and ascend to the surface of the water, in order to gain the open sea. 

 They swim in a circle ; but this pelagic life is not of long duration ; 

 they quit it after their fourth moult, which takes place between the 

 thirtieth and fortieth day, at which time they lose the transitory organs 



Fig. 333. Palinurus vulgaris. a, left outward jaw-foot. 



of natation which they have hitherto possessed. After this they are 

 no longer able to maintain themselves on the surface, but drop to the 

 bottom. Henceforth they are condemned to remain there, and such 

 walking as they can exercise beaomes their habitual mode of progres- 



