REPORT ON THE CIRRIPEDIA. 
21 
3rd. True unisexual species ; the females are large, the males very small and 
(probably) short-lived. 
Scalpellum ornatum, Gray. 
regium (Wyv. Thoms.) Hoek. 
parallelogramma, Hoek. 
nymphocola, Hoek. 
tritonis, Hoek. 
Scalpellum vitreum, Hoek. 
moluccanum, Hoek. 
eximium, Hoek. 
darwinii, Hoek. 
carinatum , Hoek, 1 &c. 
Of all the genera of Cirripedia, Scalpellum is no doubt the one which presents the 
greatest amount of variety as far as the sexual relations are concerned. In this regard 
it even surpasses the genus Ibla, Leach, of which we know, through the aid of Darwin, 
that it presents two instances of sexual differentiation only, viz., unisexuality in the one 
species and hermaphroditism with accompanying rudimentary males in the other. It is 
well known that the genus Scalpellum, by means of Scalpellum villosum, Leach, sp., and 
by means of Scalpellum trispinosum, Hoek, blends with the genus Pollicipes, Leach, 
and also that the latter genus is one of the oldest, if not the oldest, of the genera of 
Cirripedia. All the known species of Pollicipes are true hermaphrodites as are 
other Cirripedia, and, moreover, Pollicipes seems to be a genus which only contains 
shallow water species. With a little imagination it does not appear to be very 
difficult to trace the way in which sexual differentiation took place in the genus 
Scalpellum. Originally there were only hermaphrodite species, inhabitants of shallow 
water. They resembled more or less the species of the genus Pollicipes. In some of 
the species specimens attached themselves to each other 2 as well as to other objects, and 
they developed all into ordinary hermaphrodite specimens. In one of these species, 
however, young specimens attached to full-grown older ones, though developing into 
animals of the ordinary shape with a capitulum and a peduncle, did not acquire the size 
of the older specimens, and lost their female genital apparatus. In a following stage, 
we see that the little creatures which by their smallness are enabled to hide within the 
valves of the older hermaphrodite specimens, lose their valves and are reduced to a 
rudimentary state in all respects, except so far as the male organs are concerned. 
Finally, we observe in the latest stage that the original hermaphrodite specimen loses 
its male genital apparatus and becomes unisexual. In the latter species we have large 
and relatively long-lived female specimens, and small and short-lived males. 
I feel sure that some serious objections may be advanced against this reasoning, and 
1 The bodies of Scalpellum tritonis, Scalpellum vitreum, Scalpellum moluccanum, Scalpellum eximium, Scalpdlum 
darwinii, and Scalpellum carinatum, have not been investigated by means of transverse sections. Their unisexuality is 
based only on the total absence of a penis and on their general resemblance to the investigated unisexual species. 
2 Specimens of Scalpellum vulgare are attached to various horny corallines, and occasionally to the peduncles of 
other individuals. Darwin, Lepadidae, p. 226, 1851. 
