562 



SHELL. 



by the coalescence of a series of cells arranged 

 in a linear direction. They are generally 



Fig. 415. 



Tubular shell-structure from external surface of 

 Anomia Ephippium. Magnified 250 diameters. 



most abundant in shells whose exterior has a 

 foliated or sculptured character; and not un- 

 frequently they may be distinctly seen to pass 

 directly towards the prominences of the sur- 

 face, as in Lima scabra and various species 

 of Chama. They are by no means restricted, 

 however, to shells thus characterised; nor 

 are they universally present in them. 



Of the origin and mode of formation of the 

 membranous shell-structure, it is rather diffi- 

 cult to give an exact account. Possibly, after 

 the epidermic cells have undergone calcifica- 

 tion, so as to form the external cellular layer, 

 the basement membrane itself may become 

 detached from the surface of the mantle, in 

 combination with a layer of calcareous matter. 

 Even in nacre, however, which may be con- 

 sidered as the most perfect form of this sub- 

 stance, indications of cellular structure are 

 not unfrequently to be seen, especially in 

 univalve shells : these are particularly evident 

 in Haliotix, the nacreous laminae of which, 

 when carefully examined under a sufficiently 

 high magnifying power, are found to be com- 

 posed of minute cells of a long oval form 

 (fig. 416.), their short diameter not bein<* 

 above l-5000th of an inch. Their boundaries 

 in many parts are very indistinct, or even 

 disappear altogether, so that every gradation 

 can he traced, from the obviously cellular ar- 

 rangement shown in fig. 412., to the homo- 

 geneous aspect presented by the nacre of 

 bivalve shells. The same cellular structure, 

 and the same gradation to a homogeneous 

 stratum, may be made apparent in the decal- 

 cified membrane ; so that here we seem to 

 have evidence that even the membranous shell- 

 substance is originally formed by the agency 

 of cells, although the boundaries of these have 

 usually been subsequently obliterated, so that 

 the structure comes to present a homoge- 

 neous aspect. Indications of the same cel- 

 lular organisation may be detected in the na- 

 creous lining of the shell in Turho and Nautilus. 

 We seem justified in concluding that nacre 



has everywhere a similar origin ; and if one 

 variety of membranous shell-substance be thus 



Fig. 416. 



c > a 



Cellular structure of nacre ofHaliotis splendens : the 

 cells cut transversely at a, longitudinally at b, and 

 showing their terminations (with nuclear spots) 

 at c. Magnified 450 diameters. 



proved to have been formed by the agency of 

 cells, little doubt can be entertained as to" the 

 corresponding organisation of others. The 

 fact may probably be, that, as maintained by 

 Professor Goodsir *, the basement mem- 

 brane is itself composed of cells more or 

 less perfectly developed, the boundaries of 

 which usually disappear. Of this view a 

 very good illustration is afforded by the va- 

 rious examples of shell-membrane; which pre- 

 sent every gradation, from the most per- 

 fectly homogeneous pellicle, to a distinct stra- 

 tum of cells. 



The loss of the original boundaries of the 

 cells, and the consequent obscuration of the 

 real nature of the structure, are by no means 

 peculiar to shell ; for the physiologist is 

 familiar with this change as occurring in 

 various other tissues. Thus, in dentine, the 

 cases in which the vestiges of the original 

 cells are preserved are few in proportion to 

 those in which they are obliterated ; and yet 

 these isolated examples are sufficient to mark 

 the real nature of the transformation of the 

 soft dentinal pulp into the dense ivory. It 

 \yould seem as if, in the process of calcifica- 

 tion, the cell-walls have a tendency to liquefy 

 or dissolve away, unless supported by addi- 

 tional deposits of animal matter, thus allow- 

 ing the complete faxion of their contents. 

 The peculiar tenacity of the decalcified shell- 

 substance in the Margaritacece and certain 

 other tribes seems due, not so much to the 

 strength of the original, cell-walls, as to the 

 interposition of an intercellular substance 

 between them. In Pcrna we not unfre- 

 quently find, between the calcified layers, 

 membranous laminae consisting chiefly of 

 horny matter interposed between rounded 

 cells that are more or less widely separated 

 from each other : here the animal substance 



* Anatomical and Pathological Observations, p. 3, 

 note. 



