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 each 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, entermg the equalities 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 intervenmg 
