INTRODUCTORY CHAPTER. 



5 



viatile*, to those which inhabit rivers, ponds, &c., and 

 Terrestrial^, to the species which are met with on the 

 land. The most active and energetic races live only in 

 shallow water, or near the surface : those found in very 

 deep water are the lowest in their instincts, and are espe- 

 cially organised for their situation.^ 



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: the greater number, however, are con- 

 tinually moving about, and various and beautiful are the 

 contrivances by which they effect this. Some 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 ad- 

 vance, 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 protruded 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 foot for the purpose, con- 

 trive to advance with considerable agility, by coiling it up 

 tightly, and suddenly relaxing it, by which means, serving 

 as a spring, it urges the body to some distance. There 

 are many species of Mollusca which are unable to move 

 along the rough surface of the ground, but manage to 

 advance rapidly enough- on the smooth marine plants, by 

 a kind of sliding motion. Those tribes which do not 

 creep along the ground, but move through the water, 



* From Jluvius, Lat., a river. 



f From terrestris, Lat., of the land. 



j Woodward's Recent and Fossil Shells. 



B 3 



