CLAUSILIA. 179 



spiral plane, as the opening of the shell extends with 

 the growth of the animal, thus continually assimilating 

 to its size, and when the animal retreats, excluding 

 it completely from all external intrusion. In the 

 Clausilia,naiure has continued the protection afforded 

 by means of contractions and folds, and also added 

 an opercular appendage. The inhabitant of the 

 Clausilia, when nearly full-grown, secretes a thread- 

 like elastic calcareous filament, one of whose ends is 

 affixed to the columella. This filament makes half a 

 spiral turn round the columella, insinuating between 

 its folds. When the animal finishes its shell and 

 completes the aperture, it secretes, at the unattached 

 end of the filament, a spoon-shaped calcareous la- 

 mina conforming at its margin to the contour of the 

 aperture. The lamina is somewhat smaller than this, 

 and its margin is rounded. 



" Its adhesion to an elastic filament enables the 

 animal to push it, when it comes out of its shell, 

 against the columella ; and the same elasticity closes 

 it on the inhabitant retreating:, thus securino- it from 

 intruding enemies. Thus, then, this valve may be 

 compared to a door provided with an elastic spring. 

 The elasticity of the filament may be restored to its 

 full power (in the empty shell) by sometimes im- 

 mersing it in water, as I have ascertained in a section 

 made with a view to this inquiry." {Ann. Phil. iii. 

 378.) 



Draparnaud has named this valve- like appendage 

 the clausium ; and Cuvier {Regne An/maL ii. 409.) 

 states, " de cette lame on ignore I'usage dans I'animal 

 vivant." 



K 2 



