78 N. ODHNER, NORTHERN AND ARCTIC INVERTEBRATES. VI. PROSOBRANCHIA. 2. SEMIPROBOSCIDIFERA, 



short cylindrical terminal portion projects, in lengtli equal to, or shorter than, the 

 breadth of the median disc. 



The remarkable shape of the foot was pointed out by Bergh, but he did not pay atten- 

 tion to the differences between O. glacialis and O. groenlandica, though he briefly alludes 

 to them. The foot (dorsal side) consists, in specimens preserved in alcohol, of a median 

 smooth convex muscular portion extended behind and outside the visceral hump, and of 

 a marginal curling or wrinkled elevated börder. In O. glacialis the muscvilar part of the 

 foot is lengthened, everywhere decidedly broader than the side börder and like this be- 

 coming narrower backwards; itextends forwards as a broad band at the side of the visceral 

 hump (Pl. 5, Fig. 3). In O. groenlandica (Fig. 1) the foot is likewise produced backwards 

 but everywhere its central part is but little broader than the lateral börder, and in O. 

 latissima (Fig. 2) the foot end is short and flattened (length about = breadth) and its 

 central part is surrounded by a broad but flat and indistinctly defined börder, of a uni- 

 form breadth all around the foot. Besides this O. latissima differs, by having rounded 

 anterior corners of the foot, from the other species, including O. corys, which have wing- 

 like produced corners. The foot-border mentioned is apparently a contracted membra- 

 neous expansion of the foot edges, for in a specimen from Greenland (Fiskernses) the 

 foot shows partly a broad tapering margin instead of an elevated börder. 



Besides the differences in the shape of the foot the three species represent three 

 degrees as to the junction of foot and visceral hump (cfr Pl. 5, Figs. 1 — 3). In O. gla- 

 cialis the base of the visceral mäss is lengthened, narrowing backwards, in the other two 

 species it is dilated, and most so in O. latissima. 



The interiör genital apfaratus has been described by Bergh 1887 and Pelseneer 

 1895; from my own examination I have figured it (Text Fig. 5) for comparison vvith 

 Marsenina and Velutina. Some points may be given here by way of completing Bergh's 

 description. 



The narrowness of the distal female duct or the vagina in the present genus forbids 

 the supposition that it fvmctions as a female copulatory organ in consideration of the 

 large penis present in this genus. As the shape of the penis seems to correspond to the 

 form of the »Schleim- und Eiweissdriise» of Bergh, I suppose this to be a bursa copula- 

 trix {h. c), a term that I have used for it and its homologon in the preceding genera. It 

 must be remarked, however, that it is not strictly conform with the penis, for it has 

 chiefly the same construction in all species, though it is strikingly larger in O. groenlan- 

 dica than in O. latissima. It is composed of two rounded chambers, one inner larger, 

 directly debouching into the vagina close by its outer aperture and one outer one 

 smaller and communicating with the former through an opening in its lower part. 



The elements of the different parts of the genital apparatus we have found asrud- 

 iments in Velutina plicatilis (cfr Text Fig. 4 and 5). There the first stage appears of a 

 bipartition of the bursa copulatrix, and a differentiation in its frontal and under side to a 

 spermatocyst {sp. c.) as well as of a fusion of the distal ends of the vas deferens and the 

 hermaphrodite duct in close connection to this spermatocyst. The remaining part of 

 the vagina in V. plicatilis has become differentiated in Onchidiopsis into a vestibular 

 säck {v. s,, not observed by Pelseneer 1895) and a vestibular duct. 



