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. Thorns.) 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, Pollicip>es 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, Scalpellum 

 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, Lepadida^, p. 226, 1851. 



