10 
SEXUAL SELECTION. 
Part IL 
wliicli is striped, as I hear from Dr. Gunther, with bright 
colours. This filament does not contain any muscles, 
and apparently cannot be of any direct use to the fish. 
As in the case of the Callionymus, the males whilst 
young resemble in colour and structure the adult 
females. Sexual differences such as these may be 
strictly compared with those which are so frequent 
with gallinaceous birds.^^ 
Fig. 29 . Xiphopborus Hellerii. Upper figure, male ; lower figure, female. 
In a siluroid fish, inhabiting the fresh waters of South 
America, namely the Plecostomus barhatus (fig. 30), 
the male has its mouth and interoperculum fringed with 
a beard of stiff hairs, of which the female shews hardly 
a trace. These hairs are of the nature of scales. In 
another species of the same genus, soft flexible ten- 
tacles project from the front part of the head of the 
Dr. Giinther makes this remark; ‘Catalogue of Fishes in the 
British Museum/ vol. iii. 1861, p. 141, 
See Dr. Giinther on this genus, in ‘ Proc. Zoolog. Soc.’ 1868, p. 232. 
