III.] INDEPENDENT SIMILARITIES OF STRUCTUKE. 93 



small Crustacea — certain phjllopodg and ostracods — which 

 have the liard outer coat of their thorax so modified as to 

 look wonderfully 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 



CYTIIERIDKA TOROBA. 



[An ostmcwl (Cnistaccan), externally like a bivalve shellfish (Lamellibranch).] 



between the thoracic armor of the crustacean and the 

 bivalve shell, but the two sides of the ostracod and phyllo- 

 pod thorax arc connected together also by an adductor 

 muscle ! 



The pedicellarias of the echinus have been already spo- 

 ken of, and the difTiculty as to their origin from minute, 

 fortuitous, indefinite variations has been stated. But 

 structures essentially similar (called avicularia, or " bird's- 

 head processes") are developed from the surface of the 

 compound masses of certain of the highest of the pol}'])- 

 like animals (viz., the Polyzoa or, as they are sometimes 

 called, the Bryozoa). 



These compound animals have scattered over the surface 

 of their bodies minute ])rocesses, each of which is like the 



