54 CLASSIFICATION AND CREATION. 
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 
