UNIVALVE MOLLUSCANS. 277 



resembling, in miniature, a lofty tower with a 

 spiral staircase winding round it. By this atte- 

 nuated structure their motions, in their close 

 retreats, are less impeded. As it is in this tribe 

 of univalves that the organ just mentioned, the 

 operculum, or mouth-piece, first makes its ap- 

 pearance, it will not be improper here to give 

 some account of it. 



If we survey the various tribes of shell-bearing 

 animals we find them defended from the injuries 

 or attacks, to which their situation exposes them 

 by various expedients, all of them indicating 

 Power and Wisdom in their contrivance and 

 formation, and Goodness in their end. These 

 animals themselves all have a soft body fur- 

 nished with organs of different kinds, suited to 

 their station and purposes. Those that are 

 below them in the scale, especially the naked 

 Polypes, and gelatinous Radiaries, are still more 

 frail and evanescent, but their organization is so 

 inferior, that it is probably less subject to de- 

 rangement from external accidents, or injuries 

 are sooner remedied, than in that of the shell-fish 

 which, unless they were clad in some kind of 

 mail, would probably soon perish. Accordingly 

 we find some protected by a multivalve tubular 

 shell, 1 the inhabitant protruding its organs at the 

 summit, which is defended by an operculum con- 

 sisting of more than a single piece in others, 



Balcums. TMcinella. 



