flfiPORT ON THE CIERIPEDIA. 2i- 



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,' &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 Ihla, 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 ia 

 well known that the genus Scalpellum, by means of Scalpellum villosicm, 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 ^ 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 smaUness are enabled to hide within the 

 valves of the older hermaphrodite specimens, lose their valves and are reduced to a 

 rudimentary state in aU 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 



' The bodiea of Scalpellum tritonis, Scalpellum vitreum, Scalpellum moluccanum, Scalpellum eximium, Scalpellum 

 •darwinii, and Scalpellum carinatum, have not been investit,'ated by mrans of transverse sections. Their unisexuality is 

 baaed only on the total absence of a penia and on their general resemblance to the investigated unisexual species. 



' Specimens of Scalpellum vulgare arc attached to various horuy coTalliues, and occasionally to the peduncles of 

 other individuals. Darwin, Lepadidcc, p. 226, 1851. 



