54 CLASSIFICATION AND CEEATION. 



ness and vitality of their circulating fluids and 

 of the surrounding medium in which they live. 

 It consists of soft fringes, called vibratile cilia. 

 Such fringes cover the whole surface of these 

 little living beings, and by their unceasing play 

 they maintain the rotating motion that carries 

 them along in the water. 



The Mollusks, the next great division of the 

 Animal Kingdom, also include three classes. 

 With them is introduced that character of bilat- 

 eral symmetry, or division of parts on either side 

 of a longitudinal axis, that prevails throughout 

 the Animal Kingdom, with the exception of the 

 Radiates. The lowest class of Mollusks has been 

 named Acephala, to signify the absence of any 

 distinct head ; for though their whole organiza- 

 tion is based upon the principle of bilateral 

 symmetry, it is nevertheless very difficult to 

 determine which is the right side and which the 

 left in these animals, because there is so little 

 prominence in the two ends of the body that the 

 anterior and posterior extremities are hardly to 

 be distinguished. Take the oyster as an exam- 

 ple. It has, like most Acephala, a shell with two 

 valves united by a hinge on the back, one of these 

 valves being thick and swollen, while the other is 

 nearly flat. If we lift the shell, we find beneath 

 a soft lining skin covering the whole animal, and 

 called by naturalists the mantle, from the inner 



