SHELLS OF MOLLUSCA 929 



Gorgonia. They may be separated from the soft tissue in which 

 they are imbedded by means of caustic potash ; and when treated 

 with dilute acid, whereby the calcareous matter is dissolved away, 

 an organic basis is left, retaining in some degree the form of the 

 original spicule. This basis seems to be a cell in the earliest stage of 

 its formation, being an isolated particle of protoplasm without wall 

 or cavity, and the close correspondence between the appearance pre- 

 sented by thin sections of various univalve shells, and the forms of 

 the spicules of Doris, seems to justify the conclusion that even the 

 most compact shells of this group are constructed out of the like 

 elements, in a state of closer aggregation and more definite arrange- 

 ment, with the occasional occurrence of a layer of more spheroidal 

 bodies of the same kind, like those forming the vestigial shell of 

 Umax. 



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 directions for making such 

 sections which have already been given. Many of them are beautiful 

 and interesting objects for the polariscope. Much valuable informa- 

 tion may also be derived from the examination of the surfaces pre- 

 sented by fracture. The membranous residua left after the decalci- 

 fication of the shell by dilute acid may be mounted in weak spirit or 

 in Goadby's solution. 



The animals composing the class of Cephalopoda (cuttle-fish and 

 nautilus tribe) are for the most part without shells ; and the 

 structure of the few that we meet with in the genera Nautilus. Argo- 

 nauta (' paper nautilus '), and Spir-ula does not present any peculi- 

 arities that need here detain us. The rudimentary shell or scpinstnin 

 of the common cuttle-fish, however, which is frequently spoken of 

 as the ' cuttle-fish bone,' exhibits a very beautiful and remarkable 

 structure, such as causes sections of it to be very interesting micro- 

 scopic objects. The outer shelly portion of this body consists of 

 horny layers, alternating with calcified layers, in which last may be 

 seen an hexagonal arrangement somewhat corresponding with that 

 shown in fig. 698. 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 examination shows, 

 however, that, instead of a large number of detached pillars, there 

 exists a comparatively small number of very thin sinuous laminse. 

 which pass from one surface to the other, winding and doubling upon 

 themselves, so that each lamina occupies a considerable 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 arrange- 

 ment each layer is most effectually supported by those with which 



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