514 THE. OCEAN WORLD. 
The swimming of these larval creatures is produced by contrac- 
tions and expansions of the tail, and by repeated beating motions of 
the claws, the tail acting as a sort of vibratile oar, aided by which 
js vi 
: a im) 
Fig. 343.—Corystes Cassivelaunus, male. 
they maintain themselves in the water and facilitate their progress. 
As the shell becomes more solid they get less active, and finally 
return to the bottom to cast their shell and assume a new form. 
According to the observations of M. Coste, the young lobster 
casts its shell from eight to ten times in the first year, from five to 
