lii 



INTRODUCTION. 



consolidation advances, the circulation becomes move 

 and more limited in its extent, and is finally confined 

 to the base. These minute creatures, in this early state 

 of their existence, are natatory, and wonderfully active. 

 They are continually swimming from one part of the 

 vessel to the other, and when observed free in their native 

 pools, if possible even more active than when in confine- 

 ment. Their swimming is produced by continued flexions 

 and extensions of the tail, and by repeated beating mo- 

 tions of their claws ; this, together with their grotesque- 

 looking forms, gives them a most extraordinary appear- 

 ance when under examination. As the shell becomes 



Fig. rf. 



more solid they get less active, and retire to the sand 



