STRUCTURE OF THE GASTROPODS. 65 



or a pair of finely toothed jaws, or a flexible trunk, 

 which admits of being lengthened or drawn within the 

 body, the extremity of which is cleft and supplied with 

 numerous small recurved teeth capable of considerable exe- 

 cution. The common Whelk offers such an instance of 

 the retractile trunk; and the circular hole found drilled 

 in bivalve shells is due, most probably, to its destructive 

 agency. 



Concerning the muscles, it will be necessary to speak of 

 such only as serve to attach the shell. In simple univalves, 

 the muscle of adhesion sometimes encircles the back in the 

 form of a horse-shoe. This peculiarity is conspicuous in 

 Fissurella, while in the Cup-and-saucer Limpet, Cali/ptrcea, 

 it is attached to the cup-like appendage which distinguishes 

 the group. In such as are spiral, the shell is concealed by 

 a thin, riband-like muscle, attached to the axial pillar or 

 columella, by the elasticity of wliich the animal advaiices 

 its head and foot, when need requires, and again retires 

 within the last whorl. Instances, however, are not wanting, 

 where a spiral Gastropod, desirous to relinquish a portion of 

 his shell, has the faculty of sliding the attachment of the 

 columnar muscle without relaxing it, in a manner analogous 

 to the slipping of the muscular girdle in a growing Nautilus. 



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