230 THE CEPIIALOPHÖRA. ^ 203, 



substance (carbonate of lime) which effervesces with acids/^' while the oth- 

 ers enclose pigment grannies.*^"' Calcareous cells are also found, but 

 fewer, in the parts of the mantle covered by the shell. These portions of the 

 mantle serve to increase the thickness of the shell, and to repair the loss 

 of substance in places removed from the mantle-borders. 



The intimate structure of the shells of these animals is much simpler than 

 that of the Acephala. They are homogeneous throughout, and correspond to 

 the internal layer of the Bivalvia. When the carbonate of lime has been 

 extracted, the remaining organic base consists of a homogeneous membrane 

 having numerous folds varying very much as to form and number, accord- 

 ing to the genera. 



This organic base is produced by the external surface and border of 

 the mantle, in the form of a mucous liquid containing calcareous and pig- 

 ment granules, and which, hardening, forms the successive layers of the 

 shell. <ii' 



Usually there is no epidermis connecting the border of the mantle with 

 the orifice of the shell ; it can, therefore, together with the body of the 

 animal, be drawn deeply into the shell. With some Gasteropoda, how- 

 ever, 1: ; shell is covered with a kind of epidermis, which has even hair- 

 like processes.'^-' 



Many of this same order have, upon the posti " )i'sal surface of the 



foot, a peculiar plate, by which they can tiglitly close the opening of their 

 shell after having withdrawn their bodies. 



This plate, or operculum, composed, sometimes of concentric rings,, 

 and sometimes of lines spirally rolled together in the same plane, is 

 composed of a calcareous, or a horny substance, *^^' In both cases its organic 

 base is lamellated or plicated like that of the shell. The operculum [Oper- 

 culum caducum) with which certain Helicina close their shell at the begin- 

 ning of winter, is completely structureless, and without rings, spiral lines 

 or lamellae. 



Beside these external calcareous products, there are certain species of 

 these animals, which have other deposits of the same nature inside the skin 

 and in various parts of the body, which, in the form of needles, form super- 

 posed, reticulated masses.^'*' 



9 See H. Meckel, TTeber die Kalkdrüsen der !■* With Pahidina vivipara, there are, between 

 Garteuschnecke, in Miiller^s Arch. 18i6, p. 17. the cutaneous layers, numerous globular calcareous 



10 According to Gray (Lond. Med. Gaz. pt. V. bodies formed of concentric lamellae ; and with 

 18.37, 38, vol. I. p. 830), some Gasteropoda have, in Limax, not only is there a calcareous plate in the 

 the border of their mantle, numerous glands which mantle, but also a powder of the same nature scat- 

 Becrete pigment matter ; and the shell will >.. tered here and there in other parts of the skin, 

 marked according as this secretion is continuous or j no white striae which adorn the sides of the neck 

 irregular. ■mil foot of i/e/io! are composed of short, cylindri- 



11 The only solid particles I have been able to cal, thickly-set calcareous needles. 



find in this mucus are calcareous molecules which According to Kölliker, the entire skin of Poly- 



disengage gas when dissolved in acids. cera is crowded with analogous, but ramified 



In the shells of Helix, Bulimus, Cyclostoma, needles. 



Paludina, Neretina, and Cypraea, I have also Similar, probably, are the concretions which, 



been unable to find the cellular structure which with Teri^ipes, are found everywhere beneath the 



Bowerbank (Ann. of Nat. Hist. No. 68, 1843) af- skin (rfordmann, loc. cit. p. 9, Taf. III. fig. 4 a.), 



firms e.\ist in tliosc! of several Gasteropoda. and the calcareous net-work found in the mantle 



I'i Helix hir.siitii, /lixpida, villosa, and the and foot of several species of üorw {LovM, Isia,, 



young of Paliidinu XHiipara. 1S4'2, p. 361, "Vaf. I. fig. 3). 



13 The operculum is horny with Paludina, Co- 

 nus, Huccinam, Can.iis, Murex ; and calcareous 

 with Neriia, Turbo, Cyclostoma. 



