INTRODUCTORY CHAPTER. 



7 



Fluviatile*, to those which inhabit rivers, ponds, 

 &c, and Terrestrial 1[, to the species which are 

 met with on the land. 



The methods of locomotion adopted by those 

 Mollusca which move from one place to another, 

 are very various. A great number, like the 

 oyster, are perfectly stationary, and do not, 

 when once fixed, move from the place they 

 have selected; there are, however, several 

 genera that are continually moving about, and 

 various and beautiful are the contrivances by 

 which they effect this object. Many move 

 along the surface of the ground or bottom of 

 the sea, by means of their foot, which they 

 thrust out to a distance in advance, and fix to 

 some solid object, after effecting which, a 

 strong muscular contraction enables them to 

 draw their body to it ; it is then again pro- 

 truded in a similar manner, and by this means, 

 the animal moves slowly: others, using an 

 action the reverse of this, push backwards with 

 the foot, and thus impel themselves forward, 

 and others, still however making use of the 



* From fluvius, Lat., a river, 

 f From terrestris, Lat., earthly. 



B 4 



