158 
COMPARATIVE ZOOLOGY. 
for locomotion. These become reduced in number, and 
progressively perfected, as we advance in the scale of 
rank. Thus, the Infusorian is covered with thousands of 
hair-like cilia; the Star-iish has hundreds of soft, unjoint¬ 
ed, 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 loco¬ 
motion ; 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—towards 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 
progression through water, are the most perfect swimmers. 
Thus, the Salmon can go twenty miles an hour, and even 
Fig. 123—The Fins of a Fish (Pike-perch). 
ascend cataracts. They have fins of two kinds: those set 
obliquely to the body, and in pairs; and those which are 
