HOW ANIMALS MOVE. 155 
2. Locomotion.—AlIl animals have the power of vol- 
untary motion, and all, at one time or another, have the 
means of moving themselves from place to place. A few 
are free in the embryo-life, and fixed when adult, as the 
Sponge, Coral, Crinoid, and Oyster. There may be no 
regular well-defined means of progression, as in the Ame- 
ba, which extemporizes arms to creep over the surface; 
or movement may be accomplished by the contraction 
of the whole body, as in the Jelly-fish, which, pulsating 
about fifteen times in a minute, propels itself backward 
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 number, and 
progressively perfected, as we advance in the scale of 
rank. Thus, the Animalcule is covered with thousands 
of hair-like cilia; the Star-fish has hundreds of soft, un- 
jointed, 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. 
(1) 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- 
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 com- 
mon Squid on our coast admits water into the interior of 
the body, and then suddenly forces it out through a fun- 
nel, and thus moves backward, or forward, or around, ac- 
cording as the funnel is turned—toward the head, or tail, 
or to one side. The Lobster has a fin at the end of its 
tail, and propels itself backward by a quick down-stroke 
of the abdomen. 
But Fishes, whose bodies offer the least resistance to 
