228 THE MECHANICAL FUNCTIONS. 



jecting ridge which cuts its way, like a plough- 

 share, along the surface on which it moves. 

 The bands of muscular fibres, which compose 

 the principal part of its structure, are short, 

 and are interlaced together in a very intricate 

 arrangement. All the columns of their fibres 

 terminate at the surface of the disk ; so that 

 when the animal is crawling their successive 

 actions 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 corres- 

 ponding portion of the animal is dragged along, 

 so that the body advances by a slow and uni- 

 form gliding motion. The operation 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 ad- 

 vance in the direction of the motion of the snail, 

 but with twice the velocity, present a curious 

 and interesting spectacle. 



A mucilaginous secretion generally exudes 

 from the surface of the disk, and tends to in- 

 crease considerably 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 adhesion is greatly favoured by the 

 conical form of the shell, which, having a cir- 

 cular base, enables the muscles of the disk, by 



