GASTEROPODA. 167 



power of adhesion, both when the animal is crawling, and 

 also when it fixes itself on any surface. In the Patella, or 

 limpet, this adhesion is greatly favoured by the conical form 

 of the shell, which having a circular base, enables the mus- 

 cles of the disk, by their efforts to create a vacuum under- 

 neath it, to command the whole hydrostatic pressure of the 

 superincumbent water, as well as of the atmosphere above 

 the water. Besides the muscular 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 Buccinmn xindatuvi, or Whelk, is capa- 

 ble 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 Scyllsea is grooved, for the purpose of enabling the ani- 

 mal 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 tcntacula, 

 which the animal protrudes for the purpose of feeling its 

 way as it advances, and which are quickly retracted, \i^ the 

 reversion of the tube, when they arc touched or irritated. 

 This mechanism is matter of familiar observation in the tcn- 

 tacula, or horns, of the snail and of the slug, which are ter- 

 restrial mollusca belonjiins to this order. The former of 

 these has a turbinated shell of the ordinary structure: the 

 latter, though extremely similar in its internal structure to 

 the snail, is destitute of any external shell; but is furnished, 

 instead of it, with a small internal plate of cartilage, giving 

 support to some of the vital organs. 



• Osier, Phil. Trans, for 1826, p. 352. 



