ocellated, bright-coloured spots; whilst the same fin in the female is
smaller, of a different shape, and marked only with irregularly curved
brown spots. In the male the basal margin of the anal fin is also a
little produced and dark coloured. In the male of an allied form, the
Xiphophorus Hellerii (Fig. 30), the inferior margin of the caudal fin
is developed into a long filament, which, as I hear from Dr. Gunther,
is striped 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 the adult
females in colour and structure. Sexual differences such as these may
be strictly compared with those which are so frequent with gallinaceous
birds. (17. Dr. Gunther makes this remark; Catalogue of Fishes in the
British Museum, vol. iii. 1861, p. 141.)
[Fig.31. Plecostomus barbatus. Upper figure, head of male; lower
figure, female.]
In a siluroid fish, inhabiting the fresh waters of South America, the
Plecostomus barbatus (18. See Dr. Gunther on this genus, in
Proceedings of the Zoological Society, 1868, p. 232.) (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 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 probable, but we can hardly suppose that
stiff hairs and flexible filaments can be useful in any ordinary way to
the males alone. In that strange monster, the Chimaera monstrosa, the
male has a hook-shaped bone on the top of the head, directed forwards,
with its end rounded and covered with sharp spines; in the female this
crown is altogether absent, but what its use may be to the male is
utterly unknown. (19. F. Buckland, in Land and Water, July 1868, p.
377, with a figure. Many other cases could be added of structures
peculiar to the male, of which the uses are not known.)
The structures as yet referred to are permanent in the male after he
has arrived at maturity; but with some Blennies, and in another allied
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