90 



THE GENESIS OF SPECIES 



[ClIAP. 



their thorax so modified as to look exceedingly like a 

 bivalve shell, although its nature and composition are 

 ([uite different. But this is Ijy no means all, — not only 

 is there this external resemblance between the thoracic 



A POLYZOON, WITH BIRD's-UEAD PROCESSES. 



armour of the crustacean and the bivalve shell, but the 

 two sides of the ostracod and phyllopod thorax are con- 

 nected together also by an adductor muscle ! 



