402 TEGUMENTARY ORGANS 



had arisen from the coalescence of lineally arranged cells," pointed 

 out by Mr. Bowerbank and Dr. Carpenter. Having already given 

 what are, I believe, sufficient reasons for denying the existence of 

 cells of any kind in molluscan shells, I need hardly add that I cannot 

 think this to be the true explanation of the mode of development of 

 these tubules. In fact, I consider that the tubular shell structure is. 

 identical with that of dentine, and has precisely the same origin ; its. 

 tubuli arising not from cells, but like the canaliculi of bone, by a 

 process of vacuolation in the calcified tissue. I regard the structure 

 and mode of development of the Molluscan like that of the Annulose 

 shell, in fact, as evidence of the strongest and most unmistakable kind 

 in favour of the views with regard to the formation of dentine which 

 I ventured to put forth in my essay " On the Development of the 

 Teeth." Tooth and shell completely represent one another, structure 

 for structure ; Nasmyth's membrane is the homologue of the 

 " epidermis," the enamel that of the prismatic structure, the dentine,, 

 that of the membranous structure ; and all three are produced without 

 the intervention of cells by the differentiation of primarily structure- 

 less laminje. The existence of tubuli in the prismatic substance is 

 not mentioned by Dr. Carpenter, but I have noticed them very 

 distinctly in one of the sections of Pinna from his cabinet. 



Finally in Rudistes and the sessile Cirrhopods, Dr. Carpenter has. 

 pointed out the existence of a peculiar cancellated structure " like that 

 of Pinna on a large scale " only that the segments of the prisms are: 

 hollow instead of solid. These hollow prisms are covered externally 

 and internally by a structureless layer. 



To complete this view of the different varieties of shell structure,, 

 it may now be interesting to consider the mode in which they are 

 combined in the shells of the various classes of the Mollusca. In the 

 Brachiopoda, the calcareous shell is composed entirely of membranous 

 laminae, which are superimposed at a very acute angle with the surface 

 of the shell, and are further remarkable for being thrown into sharp, 

 folds -s-oVo to Toir of ^" ''""^h apart, perpendicular to their planes. In 

 the great majority of the recent species again, all the layers of the 

 shell but the outermost are perforated by canals '0006 to '0024 of an 

 inch in diameter, each of which contains a coecal process of the mantle, 

 corresponding with those processes which we have seen into the 

 cellulose tunic of the Ascidians ; the shells of Lingula and Orbicida 

 are composed of horny laminae perforated by oblique tubuli like those 

 of dentine (Carpenter, /. c). The shells of those families of Lamelli- 

 branchs, in which the lobes of the mantle are more or less united, are 

 similarly composed almost entirely of laminated membranous shell 



