III.] INDEPENDENT SIMILARITIES OF STRUCTURE. 93 



small Crustacea certain phyllopods and ostracods which 

 have the hard 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 



CTTHEEIDEA TOROSA. 



[An ostracod (Crustacean), externally like a bivalve shell-fish (Lamellibranch).] 



between the thoracic armor of the crustacean and the 

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

 pod thorax are connected together also by an adductor 

 muscle ! 



The pedicellariae of the echinus have been already spo- 

 ken of, and the difficulty 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 polyp- 

 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 processes, each of which is like the 



