SHELLS OF CEPHALOPODS. 605 



the same kind, like those forming the rudimentary shell of 

 Limax. 



469. The animals composing the Class of Cephalopoda (Cuttle- 

 fish and Nautilus tribe) are for the most part unpossessed of Shells ; 

 and the structure of the few that we meet-with in the genera 

 Nautilus, Argonauta (Paper-nautilus), and Spirilla, does not pre- 

 sent any peculiarities that need here detain ns. The rudimentary 

 Shell or Sepiostaire of the common Cuttle-fish, however, which is 

 frequently spoken-of as the Cuttle-fish Bone, exhibits a very beau- 

 tiful and remarkable structure, such us causes sections of it to be 

 very interesting Microscopic objects. The outer shelly portion of 

 this body consists of hoimy layers, alternating with calcified layers, 

 in which last may be seen a hexagonal arrangement somewhat cor- 

 responding with that in Fig. 309. The soft friable substance that 

 occupies the hollow of this boat-shaped shell, is formed of a number 

 of delicate Calcareous plates, running across it from one side to the 

 other in parallel directions, but separated by intervals several times 

 wider than the thickness of the plates ; and these intervals are in 

 great part filled-up by what appear to be fibres or slender pillars, 

 passing from one plate or floor to another. A more careful exami- 

 nation shows, however, that instead of a large number of detached 

 pillars, there exists a comparatively small number of very thin 

 sinuous lamina?, which pass from one surface to the other, winding 

 and doubling upon themselves, so that each lamina occupies a con- 

 siderable space. Their precise arrangement is best seen by examining 

 the parallel plates, after the sinuous lamina? have been detached 

 from them ; the lines of junction being distinctly indicated upon 

 these. By this arrangement each layer is most effectually supported 

 by those with which it is connected above and below ; and the 

 sinuosity of the thin intervening laminae, answering exactly the 

 same purpose as the ' corrugation ' given to Iron plates for the sake 

 of diminishing their flexibility, adds greatly to the strength of this 

 curious texture ; which is at the same time lightened by the large 

 amount of open space between the parallel plates, that intervenes 

 among the sinuosities of the lamina?. The best method of exa- 

 mining this structure, is to make Sections of it with a sharp knife 

 in various directions, taking care that the sections are no thicker 

 than is requisite for holding-together ; and these may be mounted 

 on a Black Ground as Opaque objects, or in Canada Balsam as 

 Transparent objects, under which aspect they furnish very beautiful 

 objects for the Polariscope. 



470. The structure of Shells generally is best examined by 

 making Sections in different planes, as nearly parallel as may be 

 possible to the surfaces of the shell ; and other Sections at right 

 angles to these : the former may be designated as horizontal, the 

 latter as vertical. Nothing need here be added to the full direc- 

 tions for making such Sections which have already been given 

 (§§ 138-140). Much valuable information may also be derived, 



