104 INTEOBUCTION TO CONCHOLOGY. 



teristic group,, clistinguislied for their size and muscular 

 strength^ and the voracity and comparative activity of their 

 habits. The head is furnished with a long retractile trunk, 

 capable of being rapidly protruded or withdrawn, as also 

 with an ample porous disc, which imbibes water like 

 sponge. TOien, therefore, the mollusk desires to swim, his 

 will acts upon the disc, which becomes inflated with an 

 enormous quantity of water, and the little mariner is thus 

 enabled to pass over the surface of the deep, wherever his 

 inclinations lead him. M. Deshayes frequently observed 

 this curious process on the shores of the Mediterranean, 

 during liis expedition to Algeria. 



The shell is, also, admirably adapted for aquatic excur- 

 sions, being the lightest of spiral univalves ; it approaches 

 somewhat in form to that of Cassis^ but is distinguished 

 by its thinness and want of varices, as also by the absence 

 of any recurvature at the base. A much greater variety 

 and brightness of colour is exhibited in the inhabitant, than 

 in the shell. 



The genus Magilus presents a remarkable instance of 

 intelligent economy ; it affords, also, an example of the 

 fallacy of arranging shells according to their external form 

 and aspect, without reference to the animal inhabitant. 



