258 EVENINGS AT TIIE MICROSCOPE. 



CHAPTER XV. 



WORMS. 



An examination of the diverse modes in which locomotion 

 is performed among animals, and the various organs and 

 modifications of organs that subserve this important pur- 

 pose, would form no uninteresting chapter in natural 

 history. You have two feet ; your dog has four ; in the 

 bird, two of these are converted into wings, with which it 

 rises into the air; in the fish all of them are become fins, 

 with which it strikes the water. But it is in the inverte- 

 brate classes that we discover the strongest variations. 

 The Poulpe "flops" awkwardly but vigorously along, by 

 the alternate contractions and expansions of the web that 

 unites its arm; the Snail glides evenly over the herbage 

 by means of its muscular disk; the Scallop leaps about by 

 puffs of water driven from its appressed lips; the Lobster 

 shoots several yards in a second by the blow of its tail 

 upon the water ; the Gossamer Spider floats among the 

 clouds upon a balloon that it has spun from its own body ; 

 the Centipede winds slowly along upon a " hundred" pairs 

 of feet ; the Beetle darts like an arrow upon three ; and 

 the Butterfly sails on the atmosphere with those painted 

 fans which are properly "aerial gills." How elegantly 

 does the Planaria swim by the undulation of its thin 

 body, and the Medusa by the pumping forth of the water 

 held within its umbrella ! How wondrously does the 

 Echinus glide along the side of the tank on its hundreds 

 of sucking-disks ! How beautiful, and at the same time 

 how effective, are the ciliary wheels of the Brachionus ! 



