13 



addition to the above types there are cases (Parker) where 

 there seem to be no waves at all and the foot glides like 

 a Planarian over the substratum. In Buccinum the 

 waves can only be detected at the edges of the foot, but 

 in addition to this motion the whole of the anterior part 

 can be moved forward and then attached whilst the 

 posterior portion is pulled up to it. This is particularly 

 well seen when the mollusc is removed from the water. 

 The actual forward motion of the foot is caused by 

 muscular contraction pulling forwards all those regions 

 of the foot temporarily raised, — at least when compared 

 with the rest of the foot. These regions are the waves, 

 the crests being for the time the fixed portions. 



The Operculum is a disc of chitin with a deposit of 

 calcium carbonate, placed on the dorsal surface of the 

 posterior part of the foot (PL I, fig. 7, Op., and fig. 5). 

 It is carried on a slight elevation, the " opercular disc/' 

 and when the animal is withdrawn into the shell fits into 

 the mouth, closing the orifice completely. The operculum 

 is pulled to with considerable force, for, since the colum- 

 ellar muscle is attached to this plate and the direction of 

 pull is almost exactly along the muscle, the whole force 

 of which the muscle is capable can be exerted. The 

 amount of lime in the operculum of Buccinum is but 

 small, and the structure is horny in appearance, lacking 

 the strength of some other gastropod opercula which may 

 be thick and extremely hard. 



Seen from its superior surface, it is marked by very 

 distinct lines of growth which are arranged concentrically 

 round an eccentrically placed " marginal nucleus." The 

 attachment to the foot is also eccentric, the oval area 

 lying quite to one side, the side further from the nucleus 

 and anterior when the foot is uncontracted. Round the 

 area of attachment the tissue of the opercular disc forms 



