HOW ANIMALS MOVE 367 



face ; or movement may be accomplished by the con- 

 traction of the whole body, as in the jellyfish, which, 

 pulsating about fifteen times in a minute, propels itself 

 through the water. So the worms and snakes swim by 

 the undulations of the body. 



But as a rule, animals are provided with special 

 organs for locomotion. These become reduced in num- 

 ber, and progressively perfected, as we advance in the 

 scale of rank. Thus, the infusorian is covered with 

 thousands of hairlike cilia; the starfish has hundreds 

 of soft, unjointed, tubular suckers ; the centipede has 

 from 30 to 40 jointed hollow legs; the lobster, 10; the 

 spider, 8 ; and the insect, 6 ; the quadruped has 4 solid 

 limbs for locomotion ; and man, only 2. 



(i) Locomotion in Water. As only the lower forms of 

 life are aquatic, and as the weight of the body is partly 

 sustained by the element, we must expect to find the or- 



FIG. 320. The Fins of a Fish (Pike Perch}. 



gans of progression simple and feeble. The Infusoria 

 swim with great rapidity by the incessant vibrations of 

 the delicate filaments, or cilia, on their bodies. The 

 common squid on our coast admits water into the inte- 

 rior of the body, and then suddenly forces it out through 

 a funnel, and thus moves backward, or forward, or 

 around, according as the funnel is turned toward the 

 head, or tail, or to one side. The lobster has a fin at 



