INTIMATE STRUCTURE OF THE SHELLS OF BRACHIOPODA. 29 



complete. In many Terebratulse, these disks are seen to be surrounded by delicate radiating 

 lines, as was first pointed out by Mr. Quekett ;^ the nature of these is somewhat obscure, 

 but from the appearances presented by certain preparations made from young shells by 

 Mr. Quekett, I am disposed (with him) to consider them as cilia, whose office it is to 

 produce currents of water over the extremities of the caeca, for the purpose of aerating 

 their contents. 



The case is very different, however, with the Craniadce ; for here the canals, instead of 

 enlarging as they approach the exterior, subdivide in an arborescent manner (Plate V, 

 fig. 8) ; and thus, instead of presenting on the external surface a set of definite discoidal 

 terminations, their ultimate ramifications alone are seen (PI. V, fig. 12). The largest 

 diameter of the canals is here at their internal orifices, save where these are contracted by 

 the interposition of a new internal layer (as already noticed, p. 24), between the preceding 

 and the surface of the mantle, in the manner shown in fig. 8. 



When it was first discovered that the "punctuations" which had been previously 

 noticed upon the surface of many Brachiopodous shells, are really the orifices of canals 

 penetrating the whole thickness of the shell,^ it was natural to inquire what is their 

 probable function in the economy of the animal ; and at the time when it was not known 

 that they are closed externally by a discoidal operculum, the opinion seemed not an 

 improbable one, that they are in some way connected with the function of respiration, 

 serving to introduce water into contact with the surface of the mantle which lines the 

 shell. Subsequent research,^ however, showed that they are occupied in the living animal 

 by caecal tubuli, which extend into the shell from the membranous layer that is incor- 

 porated with its inner surface. These are best seen by exposing a portion of a shell that has 

 been preserved in spirits, with the animal, to the action of dilute acid ; for attached to the 

 membranous films that form the residuum, are then found a multitude of caeca, more or less 

 crowded with yellowish-brown cells (Plate IV, fig. 9), and corresponding in size and 

 arrangement with the passages in the shell. If the process of decalcification be stopped 

 at a fitting time, a lamina of shell may be preserved, with the tubular caeca corresponding 

 to its whole previous thickness projecting from its surface (PL IV, fig. 13) ; thus conclusively 

 proving that their true place is actually in the midst of the shell-substance. Similar caeca, 

 less crowded with cells, may be readily detected in the membranous residuum of a recent 

 Crania (Plate V, fig 11). 



It is of considerable importance to the due estimation of the zoological value of the 

 character afforded by the presence or absence of the perforations, that the physiological 

 nature of these caeca should be clearly determined. I regret that I have not yet been able 



^ Histological Catalogue of the Museum of tlie Royal College of Surgeons, vol. i, p. 270. 



* This discovery was first announced in the author's Paper on the Microscopic Structure of Shell, 

 read at the Royal Society, Jan. 17, 1843, of which an abstract was published in tlie 'Ann. of Nat. Hist.,' 

 Dec, 1843. 



2 See the author's Memoir in the ' Reports of the British Association for 184",' p. 93. 



