BKACHIOPODA. 



361 



Fig. 146. Terebratula. 



it consists of flattened prisms of considerable length, arranged 

 parallel to each other with great regularity, and obliquely 

 to the surfaces of the shell, the interior of which is imbricated 

 by their out-crop (Fig. 146). This struc- 

 ture is found only in the EhynchoneUidce ; 

 but in most — perhaps all the other 

 Brachiopoda* — the shell is traversed by 

 canals from one surface to the other, 

 nearly vertically, and regularly, the dis- 

 tance and size of the perforations varying 

 with the species. Their external orifices 

 are trumpet-shaped, the inner often very 

 small ; sometimes they bifurcate towards 

 the exterior, and in Cramaihej become aborescent. The canals 

 are occupied by coecal processes of the outer mantle-layer, f 

 and are covered externally by a thickening of the epidermis. 

 Mr. Iluxley has suggested that these coeca are analogous to 

 the vascular processes by which in many ascidians the tunic 

 adheres to the test; the extent of which adhesion varies in 

 closely allied genera. The large tubular spines of the Produc- 

 tidce must have been also lined by prolongations of the mantle ; 

 but their development was more probably related to the main- 

 tenance of the shell in a fixed position, than to the internal 

 economy of the animal. (King.) Dr. Carpenter states that 

 the shell of the Brachiopoda generally contains less animal 

 matter than other bivalves ; but that Discina and Lingida con- 

 sist almost entirely of a horny animal substance, which is 

 laminar, and penetrated by oblique tubuli of extreme minute- 

 ness. He has also shown that there is not in these shells that 

 distinction between the outer and inner layers, either in struc- 

 ture or mode of growth, which prevails among the ordinary 

 bivalves ; the inner layers only difler in the minute size of the 

 perforations, and the whole thickness corresponds with the 

 outer layer only in the Lamellibranchiata. The loop, or 

 brachial processes, are always impunctate. Mr. Hancock's 

 researches would tend to show that these conclusions are gene- 

 rally correct, but not entirely so. " When the shell is dissolved 



* The fossil shells of the older rocks are so generally pseudomorphous, or partake of 

 the metamorphic character of the rock itself, that it is difficult to obtain specimens in 

 a state fit for microscopic examination. 



t Called the "lining membrane of the shell," by Dr. Carpenter. (Davidson Tntr. 

 Mon. Brach.) M. Queckett states that the perforations are closed externally by disks, 

 surrounded by radiating lines, supposed to indicate the existence of vibratile cilia in 

 the li\'ing specimens. 



B 



