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Show 1378.] SHELLS OF CEPHALOPODS. 969 As to the function or " final cause " of the chambers, I hold bv the opinion expressed in m y original memoir1 and in the * Catalogue of the Fossil Cephalopoda in the Ilunterian Museum,' 4to, ed. 18562,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 pl. vi. fig. 1, 14, of my * Memoir,' and repeats this illustration in his pl. 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 penetrating the siphon. Waagen figures the impression of these arterioles on the shell-surface4; 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 septum5. Thus, in the analysis of the structure of chambered shells, we find:-septa simple, distinct, attached only by their circumference (woodcut fig. 1, p. 966) ; 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 testaceous 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. 971), 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 L X . fig. 3, c, d). 1 "From the adhesion of the entire circumference of the mantle to the shell by means of the ' h o m y girdle '('annulus,' Waagen, op. cit.), I a m 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. 5 3 Memoires de la Soc. Linneeune cle Normandie, torn. x. 18o5, p. 17. * Waagen, op. cit. Taf. xxxix. fig. 4. 4 Op. cit. 4to, 1832, p. 47. |