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 
nature 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 securing 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 Animal, ii. 409.) 
states, de cette lame on ignore I’usage dans I’animal 
vivant.” 
