INTRODUCTORY CHAPTER. 



3 



most of them are enabled to cover themselves with ;i 

 secure, and at the same time commodious and beautiful 

 habitation, into which they can retire, by the aid of strong 

 contractile muscles, far enough to escape from danger 

 when it approaches. This outer skin is now usually called 

 the mantle, and is common to most species of mollusca. 

 As a considerable portion of these animals live entirely in 

 the water, from which they derive their nutriment, and 

 which they breathe by the aid of brancliice, resembling the 

 gills of fish, it is necessary that means should be pro- 

 vided for the admission of that fluid to the interior of the 

 body, and, in order to effect this, certain openings occur in 

 the mantle, through which the water passes and returns, 

 and by which also the head and foot, when these parts 

 exist, are put forth and drawn back at the will of the 

 animal. In univalves this mantle is in the form of a sac, 

 and has an opening in front ; in bivalves it is divided into 

 two lobes, one for each valve. 



Mollusca do not all possess heads, one entire class being 

 quite destitute ; when they do exist, they are usually pro- 

 vided with tentacula, (as in the animal of the Marginelhi) 

 (a a), by which the animals feel their way, and which 

 they have the power of easily re- 

 tracting when danger threatens ; 

 they have generally also eyes 

 ( b tj), and other organs of sense, 

 but they vary much in this re- 



, , n Marginella. 



spect, some possessing those or 



sight and smell, whilst others are devoid of both, and 

 apparently enjoy only that of touch. That of feeling 

 and sensibility to pain must be slight, or they could never 

 survive the mutilations they sometimes undergo, and they 

 have the power of reproducing parts that have been in- 

 jured, or even cut off. That limb, which, from its being 



