12 COLUMELLAR MUSCLE AND OPERCULUM. 



is rapid the muscles of the sole are alternately used on either 

 side, so that the effect of the motion is that of a pair of feet. In 

 attached shells, like Vermetus, the foot is only rudimentary and 

 serves merely as a support to the operculum. 



In retiring within the aperture of the shell the foot is generally 

 doubled upon itself across the middle,, so that its dorsal posterior 

 side, bearing the operculum, comes outermost ; but in Oliva and 

 Voluta it folds longitudinally, whilst the quadrate foot of Conns 

 is withdrawn obliquely, without folding, first the right and then 

 the left side. 



The Columellar Muscle and Operculum. 



As already stated, there is but one attachment of the proso- 

 branchiate to its shell ; namely, by means of the columellar 

 muscle, by which the inner face of the columella is directly 

 united with the posterior portion of the body of the animal. It 

 passes underneath the mantle, greatly thickening the body wall, 

 and terminates upon the inner face of the operculum, so that by 

 its contractions the operculum and shell are approximated. The 

 form of this muscle depends on that of the shell, and in the 

 conical, non-spiral shells especially, varies greatly from its 

 normal development. Thus, it is horseshoe-shaped in Capulus ; 

 it is divided into two portions, one of which lies on either side 

 of the anterior part of the animal, in Fissurella. In Haliotis the 

 animal is coiled around it, and its insertion, instead of being on 

 the columella, is on the middle of the inner wall of the shell 

 itself. 



At the ending of the columellar muscle in the dorsum of the 

 foot, its fibres are nearly vertical to the plane of the operculum, 

 which usually appears to be immediately superimposed upon 

 them : in Buccinum, however, Keferstein finds interposed a layer 

 of long cylindrical epithelial cells, with mostly distinct nuclei, 

 and long divided processes entering between the muscular fibres. 



The operculum, a cuticular development of these cells, is com- 

 posed, as may be seen in the corneous opercula of Murex, 

 Purpura, Triton, etc., of very thin superimposed layers. With 

 the microscope one may perceive in a thin section, the cylindri- 

 cal cells with their head attached to the lowermost layer; or, on 



