GENUS SCALPELLUM. 221 



the cirri, however, of S. villosum in all respects resemble 

 closely the cirri of Pollicipes sertus and P. spinosus. 



The Caudal Appendages are uniarticulate, small, and 

 clothed with spines : in S. villosum, however, differently 

 from in all other allied forms, there are no appendages. 



The Stomach, in those species which 1 opened, is desti- 

 tute of caeca. There are no filamentary appendages. 



Generative System. The ova are nearly spherical, and 

 remarkably large, as was stated to be the case in the in- 

 troductory discussion, in which the larva of S. vulgare, 

 in the first stage of development, was described : the 

 ovigerous fraena are small. The testes are large, but the 

 vesiculse seminales in some of the species extraordinarily 

 small. Scalpellum ornatum, and perhaps S. rutilum, 

 are unisexual ; the other species are hermaphrodite, but 

 most or at least some of the individuals, are furnished 

 with Complement al Males. These latter are fully de- 

 scribed under each species, so I will here only remark, 

 that S. ornatum, which alone (excepting perhaps S. rutilum) 

 is unisexual, has less claim than the other species to be 

 generically separated : we have seen also, in Ibla, that 

 similar sexual differences occur in two most closely allied 

 species. It is very singular how much more some of 

 the Males and Complemental Males in Scalpellum differ 

 from each other, than do the female and hermaphrodite 

 forms ; this seems due to the different stages of embryonic 

 development at which the males have been arrested. 

 In the males, however, of S. rostratum, S. Peronii, and 

 S. villosum, compared one with another, but not with the 

 males of the other species, the parts of the mouth and 

 apparently the cirri, resemble each other more closely, 

 than do the same organs in the hermaphrodites. At the 

 end of this genus I shall give a summary on the highly 

 remarkable sexual relations both in Scalpellum and Ibla. 



Distribution. — The species seem distributed over the whole world, but as 

 far as we can trust our present scanty materials, are most common in the 

 warmer temperate regions. The S. vulgare ranges from the Norwegian 

 seas to Naples. Most of the species are inhabitants of deep water. 



