OE, GLIMPSES BENEATH THE WATERS. 



shell-coated Molluscs, and for this purpose the 

 marine Aquarium offers many advantages. Other 

 classes of animals have been distinguished by the 

 number of their feet ; we have, for instance, a 

 tribe of worms termed centipedes, or hundred- 

 footed creatures; and, to pass over many grada- 

 tions, to the superior grass-feeding and carnivorous 

 animals, we find them termed quadrupeds, or four- 

 footed creatures; while the human race, along 

 with birds, has been termed biped. Why, there- 

 fore, may we not coin a word for our present pur- 

 pose, and call these curious Molluscs monopeds, or 

 single-footed creatures ? — for they walk with a single 

 foot, being compelled to do so by the very simple fact 

 that they have no other. This limb, or foot, being 

 gradually protruded, its bearing against some sub- 

 stance forces them forward, and when the foot has 

 attained its full distension it is drawn in, and a new 

 bearing obtained, and by the repetition of this pro- 

 cess, a certain amount of locomotion is effected. 

 Some species float on the surface by means of this 

 foot. Having crawled up a rock to the height of 

 the surface of the water, the foot is protruded and 

 exposed to the air, when it becomes suddenly dried, 

 and in that state serves as a cork, which enables 

 the animal to float away close under the surface of 



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