STRUCTURE OF SHELLS. 22.9 



their eftbrts to create a vacuum underncatli it, 

 to command the whole hydrostatic pressure of 

 the superincumbent water, as well as of the 

 atmosphere above the water. Besides the mus- 

 cular bands contained in the substance of the 

 foot, other sets of fibres are provided for the 

 purpose of protruding or of retracting the whole 

 member, and of moving it in different directions. 



The foot of the 13iiccinum undatum, or Whelk, 

 is capable of great dilatation by means of four 

 tubes, which open from the surface near the 

 gullet, and convey into it a large quantity of 

 water. It may, by this means, be distended to 

 a size even greater than the shell itself; so that 

 the opening which it forms in the sand is large 

 enough to receive the shell, when the latter is 

 drawn down by the contraction of the muscles 

 which are attached to the foot.* The foot of 

 the Scyllcea is grooved, for the purpose of 

 enabling the animal to lay hold of the stems 

 and branches of marine plants, and advance 

 along them by a gliding motion. 



The head is generally furnished with tubular 

 tentacula, which the animal protrudes for the 

 purpose of feeling its way as it advances, and 

 which are quickly retracted, by the reversion of 

 the tube, when they are touched or irritated. 

 This mechanism is matter of familiar observa- 



^ Osier, Phil. Trans, for 182(3, p. 352. 



