492 



TEGUMENTARY ORGANS. 



covered externally and internally by a struc- 

 tureless 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 vari- 

 ous classes of the Mollusca. In the Bra- 

 chiopoda, 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 remark- 

 able for being thrown into sharp folds ^Ao" 

 to yig- of an inch 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 Orbicula 

 are composed of horny laminae perforated by 

 oblique tubuli like those of dentine (Car- 

 penter, /. c.). The shells of those families of 

 Lamellibranchs, in which the lobes of the 

 mantle are more or less united, are similarly 

 composed almost entirely of laminated mem- 

 branous shell substance, e. g. Mytilus, Mo- 

 diolus, Tridacney Isocardia, Conchacece, Nym- 

 phacecs. 



The tubular structure is met with in the 

 ArcaceeSy in Lithodomus, in Cardium, and has 

 generally a marked relation with the costa- 

 tions or sculpturing of the outer surface; the 

 membranous and prismatic structures are 

 combined in the Myacece and SolenacecB, and 

 in those genera which have the lobes of the 

 mantle disunited, as Ostrea, Unio y Pinna. 



In the Gasteropoda the shell substance is 

 invariably membranous, but the laminae of 

 which the shell is composed, usually three in 

 number, are marked by parallel lines into 

 rhomboidal bodies, which are described by 

 Dr. Gray as crystals, by Messrs. Bowerbank 

 and Carpenter as elongated, mutually adhe- 

 rent cells. I believe that neither of these 

 expressions is exactly correct, but that these 

 bodies have the same origin as the prisms of 

 the lamellibranchiate shell; a conviction in 

 which I am strengthened by finding concen- 

 trically laminated bodies, like those of the 

 Lamellibranchiates, upon the inner surface of 

 the shell of Helix (fig. 313. D). 



In Patella the middle layer is composed of 

 perpendicular prisms, like those of Pinna. 

 Chiton resembles it in this respect, but the 

 outer layer is here composed of fibres parallel 

 to the surface, and is pierced by short canals. 

 In HaliotiSy calcified plaited laminae alternate 

 with structureless horny layers, in immediate 

 contact with which, says Dr. Carpenter, "is 

 a thin layer of large cells of a very pecu- 

 liar aspect." Dr. Carpenter considers that 

 the plaited laminae are cellular in this shell 

 also. 



Among the external shells of the Cephalo- 

 poda that of Nautilus has an external " cellu- 

 lar " layer as in Mya, and an internal nacreous 

 layer like that of Haliotis. 



The shells of allLamellibranchiata, Brachio- 

 poda, and of the majority of Gasteropod Ce- 

 phalophora are external, being from their very 

 origin never included in any involution of the 

 mantle. It is different, however, with certain 

 Cephalopoda and pulmonate Cephalophora, in 

 which the shell commences its development as 

 an internal organ covered over by the outermost 

 layer of the mantle, and may either remain so 

 enclosed during life (e.g. Sepia, Limax), or ulti- 

 mately become naked as in Spirilla and Clau- 

 silia. Although, however, these shells are 

 truly internal (a distinction which, as I have 

 endeavoured to show, carries with it some 

 important conclusions),* yet the careful ob- 

 servations upon their development in Sepia 

 by Kolliker, and in Clausilia by Gegenbaur, 

 appear to furnish abundant evidence that they 

 are still truly ecderonic structures, and that 

 they bear the same relation to ordinary shell 

 as a nail bears to a horny epidermis among 

 the higher animals. We know, in fact, that 

 the nail, though to all intents and purposes 

 mere cornified epidermis, is at first an internal 

 structure, being covered over by the outer 

 layers of the foetal epiderm. A nail remaining 

 so covered would correspond with the shell 

 of Limax or Sepia, while an ordinary nail 

 represents that of Clausilia. Gegenbaur, in 

 fact, has shown that the shell of the latter 

 mollusk commences at first like that of Limax 

 by the deposition of a layer of calcareous par- 

 ticles in the midst of the cellular ecderon of 

 the mantle beneath its outer layer of cells. 

 The shell of Limax goes no further than this 

 stage, while in Clausilia (and probably in Helix, 

 &c.) it gradually increases by addition to its 

 under surface, and finally bursts through the 

 cellular investment which takes no share in 

 its formation. It is the same with Sepia. 

 Here the internal shell, or sepiostaire, is com- 

 posed of two layers, a dorsal and a ventral ; 

 the former, according to Kolliker, is a thin 

 membrane composed of slightly wavy, parallel, 

 somewhat dark fibrils 0-001-2 " broad, which 

 frequently appear to be composed of still more 

 delicate fibrillae. So far as this membrane cor- 

 responds with the ventral layer, it is covered 

 on both surfaces by a thin structureless la- 

 mina of carbonate of lime, which has a pearly 

 aspect on the ventral surface where it is not 

 covered by the ventral layer ; while it is gra- 

 nular on the dorsal surface, and on the ventral, 

 where it is covered by the proper ventral layer, 

 presents ridges to which the plates of the 

 latter are attached. The thick ventral layer 

 of the sepiostaire is composed of lamellae set 

 at a very oblique angle to the dorsal layer, 

 and united together by close-set partitions 

 at right angles to their surface. Acted upon 

 by acid, this portion of the shell leaves 

 behind it a membranous skeleton of exactly 

 the same form, but presenting no further 

 structure. 



Young embryos present merely a fibrous 

 rudiment of the dorsal layer. The ventral 



* See Memoir on the Morphology of the Cepha- 

 lous Mollusca, Phil. Trans. 1852. 



