KINDS OF MOTION. 135 



the head, by bending the body into one or more curves, 

 then resting upon the tail, and extending the body, thus 

 moving forward, at eaeh step, nearly the whole length of 

 the body, or one or more of the curves into which it was 

 formed. In serpents this motion is well displayed ; and, 

 in some cases, it would appear that they are assisted in it by 

 means of their ribs, which act as feet. Among the mol- 

 lusca, and many of the annulose animals, the same kind of 

 motion is performed by alternate contractions and expan- 

 sions, laterally and longitudinally, of the whole of the body, 

 or of those parts which are appropriated to progressive mo- 

 tion. In some cases, these actions are so minute that the 

 body seems to glide along the surface with a uniform pro- 

 gress. Many of the annulosa are assisted in their pro- 

 gress, by the hairs or spines with which their bodies are 

 furnished, entering the inequalities of the surface, and pre- 

 venting a retrograde movement. In some, the body is so 

 soft and pliable, as easily to accommodate itself to the in- 

 equalities of the surface over which it glides, and derives as- 

 sistance from these in its progress. In others, there is a 

 viscous substance secreted from their bodies, which, in the 

 slug, may enable it to attach one part of its body more 

 firmly to the surface on which it is moving, while it drags 

 up the remainder to a new position. 



But there is another mode of moving analogous to walk- 

 ing, performed by means of the suckers to which we have 

 already referred. Where the organ of motion is a uniform 

 extended surface, as the foot of the limpet, the manner of ad- 

 vancing must resemble the motion of serpents. Part of the 

 foot will be detached from the surface, and form arches ; 

 while the remaining points will adhere by suction, until the 

 others reach new points of support. Where the suckers 

 are numerous, as on the belly of some caterpillars, they act 

 by alternate adhesion and separation, with an intervening 



