CiASTKROI'ODA. 205 



produce a visible undulatory motion of that surface. 

 The effect of these actions is that different parts of 

 the plane on which it moves are laid hold of in 

 succession, and each corresponding portion of the 

 animal is dragged along, so that tlie body advances 

 by a slow and uniform gliding motion. The opera- 

 tion of this mechanism may easily be seen in a snail, 

 by making it crawl on a pane of glass, and viewing 

 the movement of its disk from the other side of the 

 glass : the regular undulations which advance in 

 the direction of the motion of the snail, but with 

 much greater velocity, present a curious and inter- 

 esting spectacle. 



A mucilaginous secretion generally exudes from 

 the surface of the disk, and tends to increase con- 

 siderably its power of adhesion, both when the 

 animal is crawling, and also when it fixes itself on 

 any surface. In the Patella, or limpet, this adhe- 

 sion is greatly favoured by the conical form of the 

 shell, which, having a circular base, enables the 

 muscles of the disk, by their efforts to create a 

 vacuum underneath it, to command the whole hy- 

 drostatic pressure of the superincumbent water, as 

 well as of the atmosphere above the water. Be- 

 sides the muscular bands contained in the sub- 

 stance 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 di- 

 rections. 



The foot of the JBuccinum nudatum, 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 



