158 COMPARATIVE ZOOLOGY. 



for locomotion. These become reduced in number, and 

 progressively perfected, as we advance in the scale of 

 rank. Thus, the Inf tisorian is covered with thousands of 

 hair-like cilia ; the Star-fish 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 downward 

 and forward 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 



.* Dorsal 



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 



