230 THE CEPHALOPHORA. § 208. 
substance (carbonate of lime) which effervesces with acids, while 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 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 
shel].@ 
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. 
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shell after having withdrawn their bodies, , 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.“ 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, Ueber die Kalkdriisen der 
Gartenschnecke, in Miiller’s Arch. 1846, p. 17. 
10 According to Gray (Lond. Med. Gaz. pt. V. 
1837, 38, vol. I. p. 830), some Gasteropoda have, in 
the border of their 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 Heliz, Bulimus, Cyclostoma, 
Paludina, Neretina, and Cypraea, I have also 
been unable to find the cellular structure which 
Bowerbank (Ann. of Nat. Hist. No. 68, 1843) af- 
firms exist in those of several Gasteropoda. 
12 -Heliz hirsuta, hispida, villosa, and the 
young of Paludina vivipara. 
13 The operculum is horny with Paludina, Co- 
nus, Buccinum, Cassis, Murex ; 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 
Limaz, 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 Kélliker, the entire skin of Poly- 
cera is crowded with analogous, but ramifi 
needles. i 
Similar, probably, are the concretions which, 
with T'ergipes, are found everywhere beneath the 
skin (Vordmann, 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. (Lovén, Isis, 
1842, p. 361, Taf. I. fig. 3). 
