8 POWER OF LOCOMOTION. 



The alimentary canal varies in structure. Salivary 

 glands are generally observable ; and a liver, often of 

 considerable magnitude, is always present. Many mol- 

 lusca have peculiar secretions, as the ink of the cuttle- 

 fish. 



Some have the power of locomotion. Snails, as is 

 well known, move along the ground ; for a fleshy ex- 

 pansion under their bodies is full of muscles, which are 

 dilated or contracted at pleasure. This adheres, like 

 the sucker which the school-boy (little aware of the 

 means by which he eflfeets it) fastens to a stone, by the 

 pressure of the atmosphere above after its removal 

 from beneath ; and thus the creature advances, by fix- 

 ing the fore part to the ground, and drawing the re- 

 mainder after it. 



Excepting a few, however, among the higher orders 

 of these creatures, the mollusca are but imperfectly pre- 

 pared to move from place to place. With a due con- 

 sideration of the wisdom of God, the reason will be 

 immediately obvious ; such a power to its full extent 

 is not wanted. The greater number are intended to 

 be completely stationary. The oyster, the mussel, and 

 the limpet, for instance, usually adhere to rocks at 

 the bottom of the sea, and are consequently nou- 

 rished by the food providentially brought within their 



