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 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 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 



Pia. 127. Feet of Insects: A, Bibio fcbrilis; B, j n two claWS toothed 

 House-fly (Musca domestica); C, Water -beetle 



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 



