THE GENESIS OF SPECIES. 



[CHAP. 



their thorax so modified as to look exceedingly like a 

 bivalve shell, although its nature and composition are 

 quite different. But this is by no means all, not only 

 is there this external resemblance between the thoracic 



A POLYZOON, WITH BIKD'S-HEAD 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 ! 



