1878.] SHELLS OF CEPHALOPODS. 971 



in such a favourable state of preservation that the calcareous incrusta- 

 tion of the membranous siphon was entire, and formed the subject of 

 the continuous inflexible composite tube represented in pi. i. fio\ 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 con- 

 tents, whatever these may be proved to be, could be no longer enter- 

 tained. 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. J 



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 pene- 

 trate 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 plumine, is extra- 

 vascular, but not, therefore, extravital. The percolation by cellular 

 passages and intervals of a rarer, plasmal exudation from the vital 



Fig. 4. 



Spirilla 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 micro- 

 scopic 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 chouchinaf tissues 1 . 

 The high organization of the Cephalopods compared with other 



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." — " Recent and Fossil Cepha- 

 lopods," Geological Magazine, vol, v. p. 490. 



