6 INTRODUCTION. 



tirely consumed itj at which period the shell opens, the 

 crab takes his departure, and proceeds to make similar 

 attacks elsewhere. 



Another species, called the hermit crab, is also sup- 

 posed to be implicated in similar offensive operations 

 among univalves; for it is frequently found housed in the 

 vacant habitation of a buccinum, turbo, or nerite, and is 

 therefore suspected of having previously devoured the ani- 

 mal, and afterwards to have secured a retreat for itself in 

 the empty shell. 



Shell-fish, however, are not only exposed to the vora- 

 city of other animals, but frequently fall victims to each 

 other. The serpulse, pholades, and anomiae, affix them- 

 selves to the shells of other genera, wherein they bore a 

 small circular hole, to obtain access to the animal, which 

 they feed upon, and finally destroy. 



Shell-fish, though they in part constitute the food of 

 animals, birds, and fish, yet, in their turn, have some op- 

 portunity of retaliation, by partially destroying the float- 

 ing habitation of the universal despoiler, Man; the Pho- 

 las and Teredo navalis, in particular, frequently commit 

 such serious injury on ships, by boring into their planks, 

 as often to endanger the safety of the vessel, if not speed- 

 ily prevented in their operations. 



DIRECTIONS FOR COLLECTING SHELLS, AND ARRANGING 

 THEM SYSTEMATICALLY. 



With regard to collecting sliells, it is necessary to hint 

 to those students, who, by a residence on the sea coast, 

 may have an opportunity of forming collections for them- 



