BRACHIOPODA., 361 
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 Lhynchonellide ; 
but in most—vperhaps 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, andin Craniathey become aborescent. The canals 
are occupied by ccecal processes of the outer mantle-layer,t 
and are covered cxternally by a thickening of the epidermis. 
Mr. Huxley has suggested that these cceca 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- 
tidee 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 bivalyes ; but that Discina and Lingula 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 differ 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 
Fig. 146. Zerebratula. 
* The fossil shells of the older rocks are so generally pseudomorpkhous, or partake of 
the metamorphic character of the rock itself, that it is dificult to obtain specimens in 
A state fit for microscopic examination. 
+ Called the “lining membrane of the shell,” by Dr. Carpenter. (Davidson Intr. 
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 living specimens. 
R 
