TEGUMENTARY ORGANS 397 



XT5WT5- ii^- apart. Each prism, however, does not possess a set of 

 striae pecuHar to itself ; on the other hand, the parallel lines stretch 

 without interruption through the whole length of the prismatic layer, 

 as if the prisms were not there. A horizontal section of the prismatic 

 layer presents, as has been said, a coarse polygonal reticulation 

 corresponding with the lines of contact of the prisms. The substance 

 ■of the latter appears granular, but without any other structure in fully 

 formed portions {fig. 313. A). 



When a section of the prismatic substance is acted upon by dilute 

 acid, the calcareous matter is extracted, and a membranous frame- 

 work is left, presenting all the structural characteristics of the original 

 tissue, except that the prisms are now hollow, and from their trans- 

 verse striations have been well compared by Dr. Carpenter to the 

 scalariform ducts of plants. This membranous residuum readily tears 

 up into laminae, each of which corresponds, usually, to a number of 

 the fine horizontal striae. 



The white nacreous substance — membranous shell substance of 

 Dr. Carpenter — which constitutes the interior of the shell, presents, in 

 a vertical section, a horizontally striated appearance identical with that 

 of the prismatic layer, and when macerated in acid it breaks up into 

 corresponding laminae. In fact, if we leave out the vertical markings 

 which give rise to the appearance of prisms in the latter, the two 

 structures are identical. This point appears to me to have been 

 overlooked and to have given rise to the impression that there is 

 a much greater histological difference between the prismatic and 

 membranous substances, than really exists. The examination of 

 the line of junction of the two substances {fig. 313. B), however will 

 at once show their fundamental identity. The ends of some of the 

 prisms will be seen in fact to project beyond the others into the 

 membranous substance ; but it will be observed that the horizontal 

 lines of the latter pass without interruption through the prisms, and 

 therefore that the laminae of the two structures are identical. 



If we reduce these facts to their simplest expression, it will result 

 that these shells are composed throughout of superficial thin membranous 

 lamince, the outermost of which remains as epidermis, zvhile the inner 

 receive a deposit of calcareous salts. Next comes the question, however, 

 how are the structural differences between the prismatic and mem- 

 branous layers produced. 



Dr. Carpenter, in his well-known Essay, propounded the doctrine 

 that both varieties of shell structure are the result of the development 

 and coalescence of cells supplied by the mantle of the mollusk ; 

 these cells remaining permanently distinguishable and coalescing in 



