io8 SPECIAL CONVERGENCE 



the earthworm or that of some molluscs. The 

 latter are examples of phyletic convergence not 

 based directly on special homology ; the gizzard 

 in fishes and birds is a case of class convergence 

 based, as we have seen, upon a homogenetic 

 foundation. We may thus once more distinguish 

 clearly between convergence with homology and 

 that without it. This may seem rather para- 

 doxical, but it is true and sufficiently significant 

 to bear repetition : the pyloric division of the 

 stomach is homologous in fishes and birds ; but 

 the gizzard of fishes, which is a direct modifica- 

 tion of the pylorus, is not homologous with the 

 gizzard of birds, which is likewise a direct, though 

 independent, modification of the pylorus. 



In that rich storehouse of facts, Owen's 

 "Anatomy of Vertebrates" (1866), the gizzard 

 of the Grey Mullet (Mugil) is thus described 

 (vol. i. p. 418) : "The cardiac portion here forms 

 a long cul-de-sac ; the pyloric part is continued 

 from the cardiac end of this at right angles and 

 is of a conical figure externally ; but the cavity 

 within is reduced almost to a linear fissure by 

 the great development of the muscular parietes, 

 which are an inch thick at the base of the cone ; 

 and this part is lined by a thick horny epithelium." 

 In other cases the pylorus will be found to be 

 hard and muscular, as in the Red Mullet ( Upene- 

 oides]. In the Clupeoid genus Megalops, the 

 pylorus is a long, straight, porrect ascending tube, 



