253 



L.M.B.C. MEMOIRS 



No. XX. BUCOINUM. 



(The Whelk) 



BY 



WM. J. DAKIN, D.Sc, F.L.S. 



CLASSIFICATION. 



The whelk belongs to that class of the cephalophorous 

 Mollusca, the Gastropoda, which includes also the lim- 

 pets, land snails, and nudibranchs. The group is 

 characterised by the possession of an asymmetrical body, 

 a well developed head bearing eyes and tentacles, a foot 

 for creeping, and a shell consisting of one piece only 

 (univalve). In some cases the shell is reduced consider- 

 ably, and it may even disappear completely in the adult 

 (e.g. Nudibranchiata). 



The Gastropoda may be conveniently divided into 

 the two sub-classes: — Streptoneura and Euthyneura. 

 The first of these groups is defined by the nervous system 

 being involved in the torsion of the body so that the 

 visceral loop joining the visceral and pleural ganglia is 

 twisted into a figure of eight. The morphological right 

 side of the loop becomes carried over the alimentary 

 canal to flie topographical left side, and the left 

 half, under, to the right side. This sub-class is also 

 named Prosobranchia from the fact that, in most genera, 

 the gills lie anterior to the heart. 



The whelk is a representative of the Streptoneura, 

 and the common land snail is a type of the other group, 

 Euthyneura; the exact position of Buccinum can be seen 

 in the scheme given on the next page. 



