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Show 1878.] SHELLS OF CEPHALOPODS. 971 in such a favourable state of preservation that the calcareous incrustation of the membranous siphon was entire, and formed the subject of the continuous inflexible composite tube represented in pl. i. fig. 5, a, b, in that accomplished naturalist's memoir. The notion of the dilating and contracting action of the siphon of the Nautilus upon its contents, whatever these may be proved to be, could be no longer entertained. Vrolik, confirming by his dissection the existence of the siphonal artery, infers, like myself, the coexistence of a returning channel, although no vein with definite tunics was demonstrable in either dissection. Evidence of the capillary ramifications of the siphonal artery upon the pallial membrane lining, as periostracum, the interior of the shell-chambers has been adduced by the careful observers Keferstein and Waagen ; but such channels of vitality are not supposed to penetrate the shell itself. Molluscous shells, like avian feathers and mammalian hairs and teeth, do not receive the terminal divisions of the blood-vessels supplying their several pulps or formative organs. Ordinary or hard dentine, like conchine, piline, and pluraine, is extra-vascular, but not, therefore, extravital. The percolation by cellular passages and intervals of a rarer, plasmal exudation from the vital Fig. 4. Spirida australis. Section of part of shell, magnified. fluid renders intelligible the change and movements of pigment in the same hair and in the same feather. As the dentist distinguishes dead from living teeth, so the conchist regulates his estimate of the value of a " dead " as contrasted with a " living" shell. The estimable researches of Carpenter on the modifications of microscopic texture in shells parallel those that have demonstrated as many modifications of the microscopic channels by which the plasma percolates the dentinal as it does the chonchinal tissues1. The high organization of the Cephalopods compared with other 1 Laborious studies of this kind in quest of truth beget a modest reticence and an abstaining from such dogmatic utterances as those of the writer who " denies the possibility of the siphuncle maintaining the vitality of the shell, because it is certainly a non-vascular structure."-" Becent and Fossil Cephalopods," Geological Magazine, vol. v. p. 490. |
