PISHES. 883 



while young resemble the adult females in color and struct- 

 ure. Sexual differences such as these may be strictly com- 

 pared with those which are so frequent with gallinaceous 

 birds.* 



In a siluroid fish, inhabiting the fresh waters of South 

 America,, the Plecostomus fiarbatus\ (fig. 31), the male has 

 its mouth and inter-operculum fringed with a beard of stiff 

 hairs, of which the female shows hardly a trace. These 



Fig. 80. Xiphophorus Hellerii. Upper figure, male ; lower figure, female. 



hairs are of the nature of scales. In another species of the 

 same genus, soft flexible tentacles project from the front 

 part of the head of the male, which are absent in the 

 female. These tentacles are prolongations of the true skin, 

 and therefore are not homologous with the stiff hairs of the 

 former species; but it can hardly be doubted that both 

 serve the same purpose. What this purpose may be it is 

 difficult to conjecture; ornament does not here seem prob- 

 able, but we can hardly suppose that stiff hairs and flexible 



*Dr. Giinther makes this remark; "Catalogue of Fishes in the 

 British Museum," vol. iii, 1861, p. 141. 



f See Dr. Gtinther on this genus, in " Proc. Zoolog. Soc.," 1868, p. 

 232. 



