166 



THE MECHANICAL FUNCTIONS. 



ranean, these threads have been manufactured into gloves, 

 and other articles, which resemble silk. 



§ 3. Gasteropoda, 



The Mollusca which inhabit univalve or turbinated shells, 

 belong to the order of Gasteropoda, and have a more high- 

 ly developed organization than the Acephala. The part 

 which performs the office of a foot is a broad expansion of 

 fleshy substance, occupying nearly the whole under surface 

 of the animal, and forming a flat disk, capable of being 



applied to the plane along 

 which it moves. This is 

 seen in the Planorbis (Fig. 

 105, D.) In some species it 

 is fashioned into a project- 

 ing ridge which cuts its way, 

 like a ploughshare, 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 atM;he surface of the disk; so that when the 

 animal is crawling their successive actions produce a visible 

 undulatory motion of that surface. The efiect of these ac- 

 tions is that different parts of the plane on which it moves 

 are laid hold of in succession, and each corresponding por- 

 tion of the animal is dragged along, so that the body ad- 

 vances 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 move- 

 ment of its disk from the other side of the glass: the regu- 

 lar undulations which advance in the direction of the mo- 

 tion of the snail, but with twice the velocity, present a cu- 

 rious and interesting spectacle. 



A mucilaginous secretion generally exudes from the sur- 

 face of the disk, and tends to increase considerably its 



ceived that these threads are dried muscular fibres; an opinion which has 

 been adgpted by Blahivillc. 



