230 



THE CEPIIALOPIIORA. 



§ 203. 



substance (carbonate of lime) whicb eifervesces with acids/"' wbile the oth- 

 ers enclose pigment granules.'^"' 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 boi'der 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. '11' 



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, the shell is covered with a kind of epidermis, which has even hair- 

 like processes. '1-' 



Many of this same order have, upon the posterior dorsal surface of the 

 foot, a peculiar plate, by which they can tightly 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 shell after having withdrawn their bodies.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. 'i^' 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.*'*' 



tf See H. Meckel, Uelier die Kalkdriisen der 

 Garlcnschnecke, in Mill/er's Arch. 1846, j). 17. 



10 According to Gray (Lond. Med. Gaz. jit. V. 

 1837, 38, vol. I. p. 830), some Gasteropoda liave, in 

 the border of tlieir mantle, numerous glands which 

 secrete pigment matter ; and the shell will be 

 marked according as this secretion is continuous or 

 irregular. 



11 The only solid particles I have been able to 

 find in this mucus are calcareous molecules which 

 disengage gas when dissolved in acids. 



In the shells of Helix, Bulimus, Cyclostoma, 

 Paludina, Neretina, and Cypraea, I have also 

 been unable to find the cellular structure which 

 Bowerbank (Ann. of Nat. Hist. No. 6S, 1843) af- 

 firms e.xist in those of several Gasteropoda. 



lii Helix hirsuta, hispida, villosa, and the 

 young of Paludina vivipara. 



13 The operculum is liorny with Paludina, Co- 

 nns, Buccinam, Cassis, Mnrex ; and calcareous 

 with Nerita, Turbo, Cyclostoma. 



14 With Paludina vivipara, there are, between 

 the cutaneous layers, numerous globular calcareous 

 bodies formed of concentric lamellae ; and with 

 Limax, not only is there a calcareous plate in the 

 mantle, but also a powder of the same nature scat- 

 tered here and there in other parts of the skin. 

 The white striae which adorn the sides of the neck 

 and foot of Helix are composed of short, cylindri- 

 cal, thickly-set calcareous needles. 



According to Koltiker, the entire skin of Poly- 

 cera is crowded with analogous, but ramified 

 needles. 



Similar, probably, are the concretions which, 

 with Tergipes, are found everywhere beneath the 

 skin (rfordmann, loc. cit. p. 9, Taf. III. fig. 4 a.), 

 and the calcareous net-work found in the mantle 

 and foot of several species of Doris (Lov^n, Isis, 

 1842, p. 361, Taf. I. fig. 3). 



