CLAUSILIA. 277 



Other Mollusca have a calcareous operculum perma- 

 nently formed, which increases in thickness, and enlarges 

 on a depressed 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 C/aw5i/i«, 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 

 calcai'cous 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 apertm^e, it 

 secretes, at the unattached end of the filament, a spoon- 

 shaped calcareous lamina conforming at its margin to 

 the contour of the aperture. The lamina is somewhat 

 smaller than this, and its margin is rounded. Its ad- 

 hesion 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 re- 

 treating, 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 some- 

 times immersing it in water, as I have ascertained in a 

 section made with a view to this inquiry." 



Miiller had, nearly half a century before, accurately 

 described this singular piece of mechanism and called it 

 an ossiculum. He quaintly remarks that when the snail 

 has opened the door of its house, ^' Veneri et Cereri 

 otiosus vivit." The '^ Journal de Conchyliologie^ for 1853 

 contains an excellent article by M. Cailliaud on the sub- 

 ject, which is illustrated by admirably executed figures. 



