TEGUMENTARY ORGANS. 



489 



zone, whence the polygonal areolation ob- 

 served in it; while its colour arises partly 

 from the brown epidermis shining through, 

 partly from a slight tinge of the same kind 

 which runs through the prismatic substance, 

 and renders it distinguishable, even to the 

 naked eye, from the intensely white nacreous 

 layer. 



Thin vertical sections of these shells pre- 

 sent the following appearances under a high 

 magnifying power. The external edge is con- 

 stituted by a delicate brown band, the " epider- 

 mis," in which no structure of any kind 

 can be detected. Within this is the pris- 

 matic layer, a dense transparent substance 

 marked by strong parallel lines which run 

 perpendicularly to the surface and either 

 extend completely through the layer, or termi- 

 nate by joining some other within it. In the 

 former case, the spaces which they enclose 

 appear like the sections of prisms (of ^$ 

 of an inch, more or less, in diameter): in the 

 latter, they resemble longer or shorter cones 

 whose bases are turned outwards. A number 

 of such short cones are usually interposed 

 between those ends of the prisms which are 

 in contact with the epidermis. 



Internally, or at the line of contact of the 

 prismatic with the nacreous layer, the lines 

 either remain parallel or converge. 



The prisms are readily broken away from one 

 another, and in this case, or in a sufficiently 

 thin section of the whole layer, they are 

 seen to be traversed by very closely set pa- 

 rallel transverse lines about TH^^O ^ n - apart. 

 Each prism, however, does not possess a set of 

 striae peculiar 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 pre- 

 sents, 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 

 Ofe.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 cha- 

 racteristics 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 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 lamina?. 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 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 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 lamina?, the outermost of which 

 remains as epidermis t 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 distinguishable 

 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 184-8 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 



