4 



the invaginated tube is everted. This eversible tube is 

 the proboscis, and it can be extended about two inches 

 outside the opening of the apparent mouth. 



The Foot forms the greater part of the mass seen when 

 the animal is fully extended (PI. I, fig. 7). It has a very 

 different appearance in life from that seen even in the 

 best preserved dead specimens, where the foot is hard and 

 always somewhat contracted. It is formed of a complex 

 and powerful mass of muscles, and when fully extended 

 is perfectly smooth, without any trace of wrinkles, soft 

 and velvety to the touch, and capable of much change of 

 shape. It has a perfectly flat ventral surface, with a 

 blunt anterior and a tapering posterior end (PI. II, fig. 9). 

 The ventral surface or sole is used for creeping, but I 

 have also seen the anterior part used for holding food 

 matters. When a whelk supports itself above the water- 

 line in an aquarium tank, it does so solely by means of 

 suction. Some little force is required to detach it, but 

 the sole is simply slimy and no adhesive substance is 

 secreted. 



Upon the dorsal but posterior region of the foot is 

 situated a horny disc, the operculum, used for closing the 

 aperture of the shell when the animal is withdrawn. 



Running transversely across the anterior part of the 

 foot is the deep pedal groove. This will be described in 

 detail below, in the section on the foot. 



The Shell. 



The shell of the whelk (PI. I, figs. 1 and 2), secreted 

 by the mantle, consists of a single valve which is coiled 

 spirally owing to the varying conditions under which 

 different parts of the mantle edge secrete shell 

 substance. In most cases the twist is of such a 

 nature that if the shell is held by the apex with 

 the aperture away from the observer and turned down- 



