1878.] SHELLS OF CEPHALOPODS. 969 



As to the function or " final cause " of the chambers, I hold by 

 the opinion expressed in my original memoir 1 and in the ' Catalogue of 

 the 1'ossil Cephalopoda in the llunterian Museum,' 4to, ed. 1856 2 , viz. 

 that they so affect the specific gravity of the active, highly organized, 

 cephalopodous mollusk, as to enable it with little effort to rise, in the 

 case of the Nautilus, from its habitual position at the bottom of the 

 sea — and in the case of the Spirula, to sink from its more usual zone 

 at or near the surface, — such vertical movements being executed, like 

 the horizontal ones, by means of the hydrostatic mechanism worked 

 by the muscular forces of the mantle and funnel. 



The contents of the vacated chambers in Nautilus pompilius are 

 stated to be nitrogenous gas. Neither the contents nor the vital 

 properties of the siphuncle are yet known ; an artery and vein are 

 assigned for its life and nutrition, and to extend a low degree of the 

 vivifying influences to the shell. Vrolik confirms the existence of 

 the siphonal artery described and figured in pi. vi. fig. 1, 14, of my 

 ' Memoir,' and repeats this illustration in his pi. i. fig. 2, " i, artere 

 allant au siphon" 3 . . 



The siphonic artery sends off, according to Keferstein, branches to 

 the mantle which lines the bottom of the body-chamber before penetra- 

 ting the siphon. W.aagen figures the impression of these arterioles on 

 the shell-surface 4 ; and it has been suggested that these vessels may 

 supply, by secretion, the chamber-gas which 1 inferred might occupy 

 the space left free on the recession of the visceral sac from the 

 chamber-floor prior to the formation of a fresh septum 5 . 



Thus, in the analysis of the structure of chambered shells, we 

 find : — septa simple, distinct, attached only by their circumference 

 (woodcut fig. 1, p. 9Gb) ; septa attached, subcentrally, to each 

 other, as well as by their circumference to the shell-walls (fig. 2, 

 p. 967) ; septa (fig. 3 b, p. -907) attached marginally to the shell- 

 walls, A, and also to each other by tubular prolongations, c, with 

 an organized, vascular, membranous canal, d, traversing such tes- 

 taceous tube ; septa with a calcareous siphuncle consisting of a 

 series of superimposed, elongate, funnel-shaped tubes, with the 

 wide end directed toward the aperture of the shell, as in Spirula 

 (fig. 4, p. 9/1), or in the opposite direction, as in Bathmoceras. 

 The more complex siphons of Orthoceratites will be presently 

 referred to. Finally, we see in the existing Nautilus the shelly tube 

 interrupted, forming the " collar of the siphon," and the septa and 

 chambers traversed by a lime-coated membranous canal running 

 through the interrupted shell-tube (Plate LX. fig. 3, c, d). 



1 " From the adhesion of the entire circumference of the mantle to the shell 

 by means of the ' horny girdle ' (' annulus,' Waagen, op. cit.), I am inclined to 

 suppose that the whole of the chambers are excluded during the life-time of 

 the animal from external influence, and are filled only by exhalations or secretions 

 from the animal." — Memoir on the Nautilus (1832), p. 47. 



2 "The proportion of the air-chambers to the occupied dwelling-chamber of 

 the Nautilus is such as to render the whole animal of nearly the same specific 

 gravity as the surrounding water." — Catalogue, p. 29. 



3 Memoires de la Soc. Linneenne de Normandie, torn. x. 1855, p. 17. 



4 Waagen, op. cit. Taf. xxxix. fig. 4, 

 6 Op. cit. 4to, 1832, p. 47. 



