XXX 



INTRODUCTION. [cH. 



Cockle^ Razor-fish {Solen), and other kinds burrow in 

 sand and mud. The Scallop and Lima fill their bodies 

 with water, and then, suddenly collapsing and closing 

 their valves, execute a series of leaps or jerks, by which 

 they can traverse a considerable distance, although in 

 an irregular course. Certain small bivalves [Sphcerium 

 and Kellid), while floating on the surface of the water, 

 spin filaments which serve to moor them and prevent 

 their being drifted about. Several kinds of slug secrete 

 glutinous threads by which they suspend themselves 

 from trees and either remove from one branch to another 

 or drop safely to the ground. Thus it will be seen that 

 the Mollusca partake of all the modes of locomotion pos- 

 sessed by other animals — that they can walk like quadru- 

 peds, fly like birds, crawl like serpents, swim like fishes, 

 and even spin threads like spiders. 



The action of creeping in a snail is performed by the 

 close appression of its foot to the substance over which 

 it moves, as well as by muscular force. Any unevenness 

 in the surface to be traversed is filled up by folds of the 

 foot, which is extremely flexible. This action may be 

 compared to the application of one piece of flat glass to 

 another. Musical sounds, resembling those which are 

 given out by the Eolian harp, may be observed on a dry 

 summer evening by putting a garden-snail to crawl out- 

 side a window; and this phenomenon has frightened 

 many a timid or superstitious person, who could not 

 imagine whence these mysterious sounds proceeded. A 

 curious calculation was made by Mr. Thomas, an Ameri- 

 can conchologist, as to the rate of a snaiFs pace. He 

 found that it takes 16 days and 14 hours for a moderately 

 fast snail to do a mile. 



Food, — By far the greater part of the MoUusca are 

 zoophagous or animal- eaters. The food of the Acephala 



