$174. THE ACEPHALA. 191 
blended in a homogeneous organic base, that this last is not apparent ex- 
cept by the aid of acids, In a few only, does this organic base predomi- 
mate over the calcareous matter. 
The intimate structure of shells is quite varied,® but nearly always an 
external fibrous, and an internal lamellated layer may be distinguished by 
aid of a simple lens. The external layer appears ‘to have a crystalline 
texture, being composed of thickly-set, calcareous prisms, attached perpen- 
dicularly or obliquely upon the internal layer. These prisms, however, are 
not the result of a crystallization, but, as is shown from their development,” 
are only cells filled with lime, and if dissolved in acid, delicate prismatic 
cells remain as the organic base. The internal layer is made up of numer- 
ous superposed, non-cellular lamellae composed of the organic base, and 
arranged intricately in various ways. To the plicae thus formed, and 
‘between which the carbonate of lime is deposited, is due the pearly aspect 
of this internal layer. The relative thickness of these layers varies, 
Sometimes one, and sometimes the other, being the greater.” The exter- 
nal layer is undoubtedly secreted by'the borders of the mantle, while the 
internal is formed by a secretion of its external surface. 
The growth of the shell is not continuous, but occurs only at certain pe- 
riods of the year; hence the formation of concentric lines and furrows 
upon ‘its surface, analogous to the yearly rings of trees. ~ , 
The external layer is often colored, either uniformly throughout, or only in 
pots ; while the internal one rarely contains any pigment. By examining the 
cicatrized wounds which’ these animals acdidentally present, it will appear 
plain that this pigment is secreted by the borders of the mantle.” For, if 
these wounds are situated at a distance from these borders, the shell is 
never filled except by a layer of colorless matter. : 
In the shells of some Bivalvia there are, moreover, special, narrow ca- 
nals, which are either simple and traverse the shell obliquely from within 
outwards, or branched in a reticulated manner throughout its whole 
extent. © 
The shells are not attached to the animal except by muscular insertions 
along their borders, and by'an epidetmis belonging to the borders of the 
mantle. This epidermis, composed of a horny, yellowish-brown substance, 
stretches from the borders of the shell over its whole external surface,” and 
1 The shells of Lingula contain very little lime, and Cardiacea, and with Anomia, the fibrous 
and there is even still less in the flexible valves of layer appears to be wholly absent. 
Orbicula. 5 The formation of pearl occurs only upon the 
2 The microscopic structure of shells has of late inner surface of the mantle. It has, therefore, the 
‘been studied by several naturalists ; see Deshayes, same lamellated structure and iridescent property, 
Cyclop. of Anat. &c. I. p. 707; Shuttleworth, as the natural layer of shells. . 
weber den Bau d. Schalen, &c., in the Mittheil. d. 6 With Terebratula, these canals are quite dis 
naturforsch. Gesellsch. in Bern 18438, p.43; and tinct — occupying the whole thickness of the shell. 
Carpenter, Annals of Nat. Hist. XII. 1843, p. Ihave observed the same arrangement with Cy- 
373, Pl. XITI. XIV. and especially the Rep. of the c/as, while with Lingua, they are confined to the- 
Brit. Assoc. 1844, p. 1, with many figures.* internal layer. By direct light they appear black. 
8 Mya arenaria forms an exception to this; the Iam yet uncertain whether this color is due.te 
tooth of its shell contains true prismaticcrystals their extreme tenuity, or to calcareous matter in 
bound together in a star-like manner ; see Car- their interior. If the first, they would be compar- 
penter Annals of Nat. Hist. loc. cit. Pl. XIV. able to the canaliculi of the dentine of teeth ; but if 
fig. 8. the second, to the corpuscles of bone. Carpenter 
4 These two layers, of which the outer one quite (Annals of Nat. Hist. loc. cit. p. 884, Pl. XIII. fig. 
resembles the enamel of the teeth, are very dis- 5), has observed that in the shells of Lima rudis, 
tinctly seen with Malleus, Perna, Crenatula, those canals are divided and form a kind of net- 
Avicula, Meleagrina, Pinna, Anodonta, Unio, work. . . 
&c. With Ostrea, and Chama, they alternate 7 See Mytilus, Anodonta, Unio, Solen, Lutra- 
-with each other several times. In many Pectinea, ria and Mya. 
* (§ 174, note 2.] For the complete labors of Physiol. Art. Shed/, IV. p. 556. It is replete with 
Carpenter in this direction, see Cyclop. Anat. and figures. — Ep. 
