432 Miscellaneous. 



We may sometimes succeed in killing it in this position, and then 

 make antero-posterior sections embracing at once both the apparatus 

 and the cuticle in course of formation. In some of these sections 

 we have seen a streak of matter issue from the groove and become 

 impregnated, above the epithelial organ, with granules escaping 

 from the cells of the latter, the function of which would appear to 

 be to consolidate the membrane secreted by the glandular caeca. 



It is well known that the principal character of the adult Helices 

 consists in the turning-out of the margin of the shell. According 

 to the preceding statements, this eversion is explained as follows : — 

 After the atrophy of the epithelial organ the portion of the mantle 

 which it occupied sinks down, and in this depression carries with it 

 the part of the cuticle which covered it. The anterior portion of 

 this membrane situated above the pallial groove, the level of which 

 has not changed, is thus raised, and is soon lined on its inner surface 

 with the calcareous deposits which this region has acquired the 

 power of secreting. 



All the parts of the mantle posterior to those we have just been 

 considering assist in the formation of the nacre. As regards the 

 primitive forms under which the constituent elements of the various 

 calcareous layers are deposited, we shall limit ourselves to the fol- 

 lowing remarks. If we examine a fragment of the delicate mem- 

 brane detached from the margin of a shell in process of growth, we 

 observe that the most recent part is formed almost exclusively by 

 the cuticle ; a little further back this is furnished with lines of 

 spherical granules ; further still these granules form a continuous 

 calcareous layer lining the inner surface of the cuticle. In a young 

 Helix from which a portion of the cutogenic apparatus has been 

 removed, the outermost layer above the iujured spot is formed in a 

 different manner. We first of all see calcareous rods appear ; and 

 these gradually become enlarged at the two extremities, and acquire 

 the form of small wallets (or saddle-bags). By the addition of fresh 

 particles these wallets are converted into spheres, the dimensions of 

 which are increased by concentric deposits with radiating strise. 

 These spheres finally come into contact with each other. The first 

 calcareous stratum therefore is formed differently according as it is 

 uncovered or protected by the cuticle. It is also in the guise of 

 rods and spheres that the elements of the outermost layer are depo- 

 sited in the cicatrical fragments consequent on denudations, both 

 in the adult and in the young animal. This is the case also in the 

 epiphragms, but with this difference, that granules poured forth by 

 the calcareous glands of the collar are here mingled with the 

 spheres with concentric layers. We have ascertained that the 

 cicatrices of the shell are produced even when the part laid bare is 

 covered immediately with a fragment corresponding to that which 

 has been removed. Fine plates of mica introduced between the 

 the shell and the mantle are lined on the inner surface with a cal- 

 careous deposit. — Comptes Rendus, April 12, 1880, p. 882. 



