340 Natural Histoiy of Molluscous Animals : — 



reous operculum previously to hybernation, is unable to make 

 any thing more than a membranous substitute " when deprived 

 of nourishment ; " and that the snail itself thrives better, and 

 is found in greater numbers, in chalky districts than elsewhere. 

 This may all be true, and yet it proves little ; for we are far 

 from denying that the food is the source of the lime ; and 

 that lime is not necessary to be eaten in order to the perfect 

 formation of the operculum is proved, as Mr. Bell remarks, 

 by many snails in his possession having formed that part, 

 though during the whole summer they had no access to any 

 preparation of lime. * And, further, sea- water and sea-weed 

 contain a mere fraction of calcareous matter ; yet what and 

 how many large and compact shells are produced by creatures 

 which have no other sustenance ! I repeat, then, that though 

 the food unquestionably is the source of the material of shells^ 

 yet has the architect as certainly the power of changing its 

 constituent principles, and of altering their qualities. 



The mode in which the shell is built is less a mystery; so 

 that the second question admits a more satisfactory answer. 

 If you will examine the snail of any common Helix, you will 

 perceive that where the body rises into the shell there is a 

 fold or membrane of a semicircular shape. This part is 

 denominated the collar, from the manner in which it surrounds 

 the body, and it is the organ which secretes the shell. The 

 animal is born with the rudiments of its future covering, and 

 by its gradual increase of growth is enabled to push the col- 

 lar for a space, and from time to time, beyond the original 

 margin. In these operations a thin layer of membranous and 

 calcareous matter is excreted and deposited, which is gradually 

 thickened by successive layers being laid on within the first 

 by the repeated protrusions and retractions of the collar. 

 This portion being formed, the animal commences another, 

 and finishes it in the same manner ; and the extent of each 

 portion is marked in some shells by an elevated rib, in others 

 by a slight depression. There is not, as the language of some 

 authors would seem to imply, a regular and alternate deposi- 

 tion of a layer of membrane and a layer of lime ; but in all 

 shells the animal and earthy matters are obviously secreted 

 and deposited at the same moment and in commixture. And 

 although it be true that the exterior or smallest layers are 

 formed first, and that others are successively deposited sub- 

 jacent to them, yet it does not follow, as is commonly stated, 

 that each new layer is complete, and extends beyond the mar- 

 gin of the former one, so that the shell, by every addition, 



* Zoological Joiu'nal, i. 96-7. 



