TEGUMENTARY ORGANS 403 



substance, e.g. Mytilus, Modiolus, Tridacne, Isocardia, Conchacece 

 NympIiacecB. 



The tubular structure is met with in the Arcacece, in Lithodonms, 

 in Cardiiini, and has generally a marked relation with the costations 

 or sculpturing of the outer surface ; the membranous and prismatic 

 •structures are combined in the MyacecE and Soknacea, and in those 

 genera which have the lobes of the mantle disunited, as Ostrea, Unio, 

 Pinna. 



In the Gasteropoda the shell substance is invariably membranous, 

 but the laminse 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 adherent 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 concentrically 

 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 Haliotis, 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 peculiar aspect." Dr. 

 Carpenter considers that the plaited laminae are cellular in this shell also. 



Among the external shells of the Cephalopoda that of jVrtw^z'&j 

 has an external " cellular " layer as in Mya, and an internal nacreous 

 layer like that of Haliotis. 



The shells of all Lamellibranchiata, Brachiopoda, and of the 

 majority of Gasteropod Cephalophora are external, being from their 

 very origin never included in any involution of the mantle. It is 

 different, however, with certain Cephalopoda and pulmonate Cephalo- 

 phora, 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 ultimately 

 become naked as in Spirula and Clausilia. 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 

 observations 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 



' See Memoir on the Morphology of the Cephalous Mollusca, Phil. Trans. 1852. 



D D 2 



