REPORT ON THE PTEROPODA. 
7 
asymmetry of the Mollusc, any more than this latter enables us to determine the 
direction of the spiral (in contradistinction to the opinion of Lacaze Duthiers 4 ) ; hence 
an animal with dextral asymmetry may be twisted directly or inversely, or may not 
be twisted at all (Patelloid Gastropoda). 
Besides, the direction of the spiral must be of very slight morphological importance, 
since in the same genus ( Xeptunea , Pyrula, Vertigo, &c.) there are some species which 
are dextral, and others which are normally sinistral. 
1. Limacina. 
The Head is distinct, surrounded on each side by the fins, which reach to the dorsal 
aspect, where is found a pair of tentacles. 
These latter are asymmetrical, the left being much less developed than the right ; in 
Limacina lielicina and Limacina antarctica it is almost completely atrophied, and is 
situated rather posteriorly to the right. In Limacina injlata and Limacina lesueuri it 
is somewhat larger. In all the Limacinae the right tentacle is very long when fully 
developed (PI. I. fig. 1, a), and is surrounded at the base by a short everted sheath, 
such as is found in certain Xudibranchs. This sheath also occurs in the smaller forms 
(“ Spirialis”), where Souleyet 2 did not succeed in finding it. 
The Foot. — The fins form at the anterior extremity of the body a natatory surface, 
oblique with respect to the transverse plane of the body, the ventral margin being lower 
than the dorsal. On this surface the mouth opens, and at the right, near the dorsal 
margin outside the lip, is the orifice of the copulatory organ, to which the seminal groove 
leads, passing from the right side of the cervical region over the dorsal border of the fin. 
On the dorsal margin of the fin, near the middle, is a little tentaculiform lobe, which 
differs in structure from the remainder of the fin, and contains the termination of a 
slender nerve ; probably it is a tactile organ. This lobe or papilla does not exist in 
all species. I have only demonstrated its presence in Limacina helicina, Lidnacina 
antarctica, and Limacina australis; it is wanting on the other hand in Limacina 
hulimoides, Limacina trochiformis, Limacina lesueuri, and Limacina injlata. 
This little lobe corresponds to an analogous organ found in the subgenus Creseis, of 
the genus Clio. Van Beneden 3 regarded it as a tentacle, and Huxley 4 identified it with 
the long cephalic appendage of Halopsyche. These two opinions are both erroneous. 
The fins are continuous and united ventrally by the posterior lobe of the foot, which 
is slightly notched in the middle of its ventral border, and carries the operculum. I have 
] Du systeme nerveux des Gasteropodes pulmones aquatiques, Arch, de Zool. Expir., ser. 1, t. i. p. 462. 
2 Voyage de la Bonite, Zoologie, t. ii. p. 209. 
3 Memoire sur la Limacina arctica, p. 3, Mem. Acad. Sci. Bruxelles, t. xiv. 
4 On the Morphology of the Cephalous Mollusca, Phil. Trans., 1853, p. 41. 
