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 cha racteristics of the original tissue, except that the prisms are now hollow, and frorn their trans verse striations have been well compared by Dr.Carpen ter to the scalariform ducts of plants. This membranous residuum readily tears up into lamin, each of which corresponds, usually, to a number of the fine horizontal striw.
The white nacreous substance — membra neous 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 laminm. In fact, if we leave out the vertical markings which give rise to the appearance of prisms in the latter, the m o 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 sub stances, 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 sub stance; but it will be observed that the hori zontal lines of the latter pass without interruption through the prisms, and therefore that the larninx of the two structures are iden tical.
If we reduce these facts to their simplest expression, it will result that these shells are composed throughout of superficial thin membranous &mince, the outermost of which remains as epidernzis, while the inner receive a deposit of calcareous salts. Next comes the question, however, how are the structural dif ferences between the prismatic and membra nous 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 sup plied by the mantle of the mollusk ; these cells remaining permanently disting,uishable and coalescing in rows, in the prismatic struc ture, but bursting and becoming confused into a homogeneous tissue, in the membranous substance. Nor, indeed, would it have been
very easy in 1848 to arrive at any other con clusion than this, to which so great a number of appearances at first sight tend. Enabled, however, by Dr. Carpenter's great kindness and liberality to form my own judgment from his beautiful preparations, and having also worked over the fresh shells for myself, I have come to very different conclusions. I will not say that occasionally cells may not be en closed in shell, but I believe I am in a posi tion to show that, as a rule, shell-growth is not a case of conversion, but one of excretion, cells not being in any way directly concerned in the matter.
We may consider, first, the growth of the shell as a whole ; and, secondly, that of its three constituents. Inasmuch as we know, that the shell of the young Unio or Anodon was once as thin as, or thinner than, the " epidermis" of the adult shell, and smaller than the smallest area, bounded by a concen tric line on its outer surface ; further, since we know that no addition is made to the outer surface of the shell directly; it is clear that the shell must grow in size by addition to its margin ; in thickness, by addition to its under surface. Furthermore, since the ex treme margin of any shell is constituted by the horny " epiderm," internal to which is the gradually thickening layer of prismatic sub stance, constituting the brown zone, within which again is the white nacreous area, formed by the superposition of membranous layers over the fully-formed thick prismatic sub stance ; from all this, it appears to be equally certain that any given spot of the mantle of a young bivalve must give origin, directly or indirectly, first, to " epiderm ;" secondly, to prismatic substance ; and, thirdly, to nacreous substance ; so that, on examining the free edge of a growing shell, we ought, since the " epiderm" is structureless and transparent, to be able to observe the gradual formation of the prismatic substance upon its under sur face. This is, in fact, the case. Fig. 313, A, represents such a free edge of the shell of Anodon, a being the direction of the flexible zone ; b, that of the perfect prismatic sub stance.