III.] INDEPENDENT SIMILARITIES OF STRUCTURE. 91 



The pedicellaripe of the echinus have been akeady 

 spoken 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 some- 

 times called, the Bryozoa. 



These compound animals have scattered over the surface 

 of their bodies minute processes, each of which is like the 

 head of a bird, with an upper and lower beak, the whole 

 supported on a slender neck. The beak opens and shuts 

 at intervals, like the jaws of the pedicellaripe of the echinus, 



BIRD S-HEAD PROCESSES VERY GREATLY ENLARGED. 



and there is altogether, in general principle, a remarkable 

 similarity between the structures. Yet the echinus can 

 have, at most, none but the most distant genetic relation- 

 ship with the Polyzoa. We have here again therefore 

 complex and similar organs of diverse and independent 

 origin. 



In the higliest class of animals (the Mammalia) we have 

 almost always a placental mode of reproduction, i.e. the 

 blood of the foetus is placed in nutritive relation with the 

 blood of the mother by means of vascular prominences. 

 Xo trace of such a structure exists in any bird or in 

 any reptile, and yet it reappears in certain sharks. There 



