SHELLS AND MOLLUSCOUS ANIMALS. 193 



them ; and thus the animalcules, with which the 

 water is full, are brought to the lips of the animal, 

 which are so sensitive, that they admit or reject 

 what would nourish or what would Ibe useless. 

 Small as the cilia are, they are visible only when 

 seen by the microscope, and none except those 

 accustomed to make microscopic observations can 

 imagine their use, not to molluscous animals 

 merely, but to zoophytes, jelly fish, and a vast 

 number of marine creatures. The strong and 

 rapid currents which these cilia make in the water 

 cannot be imagined by those who have not wit- 

 nessed the little whirlpools caused by their perpe- 

 tual motion. Even after the shell is closed over 

 water previously admitted, their vigorous action 

 never ceases ; and when a portion of the branchiae 

 is cut off, still the cilia move up and down, as 

 long as any vitality remains, and, acting as paddles 

 to the part on which they grow, they row it through 

 the water so quickly, as that one might fancy the 

 detached piece to be a separate and living animal. 

 In the mass of the body lie the heart, the stomach, 

 the liver, and other viscera ; and such, with some 

 slight variations, is the general structure of the 

 great number of bivalve mollusks. 



That beautiful mother-of-pearl, or nacre, of 

 which so many ornamental objects are made, with 

 its glistening silvery surface, tinted with rainbow 

 hues, consists of layers of membrane in conjunction 

 with particles of lime. These bright colours have 

 been proved to be the effect of the parallel grooves 

 which are caused by the regular arrangement of 

 the successive deposits of shells. A similar effect 

 may be produced with other materials similarly 

 arranged, and casts of the surface of mother-of- 

 



