162 
COMPARATIVE ZOOLOGY. 
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 nu-| 
merous minute spines pointing backward; while the Cat-' 
erpillar 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 curving thej 
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 modified into 
pincers for prehen¬ 
sion. With the rest, 
which end in a sharp 
claw, the Crab moves 
backward, forward, 
or sideways. The 
Spider has eight legs, 
usually seven-joint¬ 
ed, and terminating 
Fig. 127. —Feet of Insects: A, Bibio febrilis; B, } n f wn 0 1 » W q fnntliml 
House-fly (Musca domestica ); C, Water - beetle 1 ° Clcl ^ S tOOtlied 
(Dytiscus). like a 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 and sec¬ 
ond left together; and lastly, the third left and second 
right together. The front and hind pairs are, therefore,, 
moved like those of a quadruped. The Insect has six 
