HOW ANIMALS MOVE. 159 
has a sucker at each end: fixing itself by the one on its 
tail, and then stretching the body, by contracting the 
muscular fibres which run around it, the creature fastens 
its mouth by suction, and draws forward the hinder parts 
by the contraction of longitudinal muscles. The Earth- 
worm lengthens and shortens itself in the same way as 
the Leech, but instead of suckers for holding its position, 
it has numerous minute spines pointing backward; while 
the Caterpillar has short legs for the same purpose. The 
legless Serpent moves by means of the scutes, or large 
scales, on the under side of the body, acted upon by the 
ribs. In a straight line, locomotion is slow; but by cury- 
ing the body laterally or vertically, it can glide or leap 
with great rapidity. 
Most animals have movable jointed limbs, acted upon 
as levers by numerous muscles. The Centipede has forty- 
two legs, each with five joints and a claw. The Crab has 
five pairs of six-jointed legs; but the front pair is modi- 
fied into pincers for prehension. With the rest, which 
end in a sharp claw, the Crab moves backward, forward, 
or sideways. The Spider has eight legs, usually seven- 
jointed, and terminating in two claws toothed like a 
a 
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Fig. 125.—Diagrammatic section of Star-fish: a, mouth; b, stomach ; ec, cecum, or in- 
testine; d, dorsal surface; e, ambulacral plates; f, ovarium; g, tubular feet; h, 
internal sacs for extending the feet. 
comb, and a third which acts like a thumb. In running, 
it moves the first right leg, then the fourth left; next, the 
first left, and then the fourth right; then the third right 
