10 



further governed by an architecture already present in 

 the secreting area of the mantle. Such a differentiation 

 of the secreting cells is, however, ultra-microscopic, and 

 the sculpture of a mollusc shell belongs to the same 

 category as the regular shape of the teeth on the radula, 

 the form of placoid scales and the growth of the Echinus 

 plates. 



The Columellar muscle is attached to the shell by the 

 same kind of cells that are noticed in Pecten and other 

 lamellibranchs. During life a movement of the muscle 

 takes place, but there is no actual movement of the 

 muscle fibres. A growth of new fibres takes place in front, 

 and resorption behind, so that as a result the whole 

 structure appears to move forwards. 



The Foot. 



The ventral creeping foot is exceedingly well 

 developed in Buccinum. It is muscular, and capable of 

 very considerable expansion and contraction, so that 

 whilst soft and almost translucent when expanded, it 

 becomes opaque and tough as cartilage and difficult to 

 deal with in dissections when contracted. The flat and 

 smooth ventral surface, or sole, has, when the foot is 

 expanded the shape indicated in fig. 9, PL II. Thus the 

 anterior margin of the foot is broad, and the width 

 remains constant for some distance backwards until 

 towards the posterior end it gradually narrows away to a 

 point. Running parallel to the anterior margin of the 

 foot is a deep groove (fig. 9, PL II, Fed. gl.), which cuts 

 off an anterior narrow ridge from the major portion of the 

 foot; and into this anterior groove open numerous glands. 

 The molluscan foot is usually well provided with glands, 

 and these may be divided into (a) scattered gland cells 

 covering the foot, and (6) specialised compound glands. 



