TEGUMENTARY ORGANS. 



491 



of the epidermis of the mantle.* The outer 

 laminae retain their membranous nature, only 

 becoming so far altered as to assume the 

 horny aspect of the so called " epidermis ; " 

 in the next laminae, which are added to the 

 inner surface of the young shell, calcareous 

 matter is deposited in granules, additions to 

 which are made in such a manner as to con- 

 stitute the celUeform concretions, and ulti- 

 mately, the process going on in the same way 

 in successive layers, the prisms ; in the inner- 

 most laminae, finally, the calcareous deposit 

 results in an even, homogeneous, folded or 

 striated layer. By scraping with a sharp knife 

 the inner surface of the shell of Anodon, 

 freshly detached from the mantle, I have 

 obtained a distinct tough membranous layer, 

 scattered through which were a vast number 

 of close-set irregular granules of calcareous 

 matter. A similar structureless layer without 

 the granules constitutes the outermost surface 

 of the ecderon of the mantle (Jig. 313. c, b') 

 and may occasionally be detached as such. 

 Such a layer consisting of the thickened outer 

 portion of the periplast of the ecderon of the 

 mantle is by no means an anomalous structure, 

 as we have a formation of exactly the same 

 kind in the " cuticle " of plants, and in the 

 chitinous lining of the intestine in Insects ; 

 and I believe that the shells of mollusks in 

 general consist simply of a multitude of thin 

 layers successively thrown off, super-imposed 

 and coherent, all the peculiarities of their struc- 

 ture arising from subsequent modifications, 

 which are altogether independent of cells. 

 This view is in perfect agreement with all 

 that is known of the nature of the shells of 

 larval Gasteropods and Acephala, which are 

 invariably either of an absolutely structureless, 

 thin, transparent, membranous character, or 

 at most present a delicate striation. It may 

 be added that not the slightest trace of a cel- 

 lular structure is to be met with in the pellucid 

 shells of the Heteropoda and Pteropoda. So 

 much for the two primary forms of shell struc- 

 ture, the membranous and the prismatic. A 

 most interesting variety of the former is 

 the nacreous (mother-of-pearl) lining which 

 is presented by many shells, both of Ace- 

 phala and Cephalophora. The pearly iri- 

 descence proceeds, as Dr. Carpenter has well 

 shown, from the folding of the membranous 

 layer into close plaits, and not, as has been 

 supposed, from the alternate cropping out of 

 calcareous and membranous layers. Dr. Car- 

 penter proved this by decalcifying with acid 

 a layer of nacre from Haliotis splendens. 

 The iridescence remained; but if the plaits of 

 the layer were pulled out by stretching it 

 with needles, the iridescence disappeared. 



Another variety of structure usually, but 

 not alone found in the membranous shell 

 substance, is the tubular. " All the different 



* This is, after all, only a return to the opinion 

 of Poli, whose observations on shell structure are 

 remarkably accurate, and should never be over- 

 looked. See his Testacea utriusque Sieilire. Pars 

 prima "in qua de Testarum natura atque aftec- 

 tionibus disputatur." 



forms of membranous shell structure are oc- 

 casionally traversed by tubes which seem to 

 commence from the inner surface of the shell, 

 and to be distributed to its several layers. 

 These tubes vary in size from about the TrrJj^ 

 to the ^TjVrr f an inch, but their general 

 diameter in the shells in which they most 

 abound is about -g-^Vo of an inch. The di- 

 rection and distribution of these tubes are 

 extremely various in different shells; in 

 general, when they exist in considerable 

 numbers, they form a network which spreads 

 itself out in each layer nearly parallel to its 

 surface, so that a large part of it conies into 

 focus at the same time in a section which 

 passes in the plane of the lamina. From this 

 network some branches proceed towards the 

 nearer side of the section as if to join the 

 network of another layer, whilst others dip 

 downwards, as if for a similar purpose" (Car- 

 penter, /. c. p. 14k). In other instances the 

 tubes run obliquely through all the layers. The 

 former structure was found by Dr. Carpenter 

 in the outer yellow layer of Anomia cpliip- 

 pium ; the outer layer of Lima scabra and in 

 Chama, the latter in Area, Pectunculus, and 

 Trigonia. In the latter case, the tubules 

 are not continuous, but are seen under a 

 high power to be formed by rows of isolated 

 vacuities, one for each lamina; corresponding, 

 I imagine, with the appearance, " as if they 

 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 con- 

 sider that the tubular shell structure is iden- 

 tical 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 inter- 

 vention of cells by the differentiation of pri- 

 marily structureless laminae. The existence 

 of tubuli in the prismatic substance is not 

 mentioned by Dr. Carpenter, but I have no- 

 ticed them very distinctly in one of the sec- 

 tions of Pinna from his cabinet. 



Finally in Rudistes and the sessile Cirrho- 

 pods, Dr. Carpenter has pointed out the ex- 

 istence 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 arc 



