^ 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- 

 nate 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 

 spots ; while the internal one rarely contains any pigment. By examining the 

 cicatrized wounds which these animals accidentally 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 epidermis 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 Lingtila contain very little lime, and Cardiacea, and with Anomia, the fibrous 



and there is even still less in the flexible valves of layei- appears to be wholly absent. 



Orbicula, 5 The formation of pearl occurs only upon the 



Ü 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 lameilated structure and iridescent property, 



Cj'clop. of Anat. &c. I. p. 707 ; Skuttleworth, as the natural layer of shells. 



•leber den Bau d. Schalen, &c., in the Mittheil. d. 6 With Terebratula, these canals are quite dis- 



naturforsch. Gesellsch. in Bern 1843, p. 43 ; and tinct — occupying the whole thickness of the shell. 



Carpenter, Annals of Nat. Hist. XII. 1843, p. I have observed the same arrangement with Cy- 



S73, PI. XIII. XIV. and especially the Rep. of the clas, while with Lingula, they are confined to the 



Brit. Assoc. 1844, p. 1, with many figures.* internal layer. By direct light they appear black. 



3 Mya arenaria forms an exception to this ; the I am yet uncertain whether this color is due to 



tooth of its shell contains true prismatic crystals 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- 



fentei Annals of Nat. Hist. loc. cit. PI. XIV. able to the canaliculi of the dentine of teeth ; but if 



fig. 8. the second, to the corpuscles of bone. Carpenter 



i These two layers, of which the outer one quite (Annals of Nat. Hist. loc. cit. p. 384, PI. XIII. fig. 



resembles the enamel of the teeth, are very dis- 5), has observed that in the shells of Z/iWia rudis, 



tinctly seen with Malleus, Perna, Crcnatula, those canals are divided and form a kind of net- 



Avicula, Meleagrina, Pinna, Anodonta, Unio, work. 



■&C. With Oslrea, and C/iama, they alternate 7 See Mytilus, Anodonta, Unio, Solen, Luira- 



with each other several times. In many Pectinea, ria and Mya. 



* [ § 174, note 2.] For the complete labors of Physiol. Art. Shell, TV. p. 556. It is replete with 

 Carpenter in this direction, see Cyclop. Anat. and figures. — Ed. 



