THE CEPHALOPODA. 



175 



The Nautilus passes the greatest portion of its life at the bed of the sea, but it has 

 been seen occasionally at the surface, and the question naturally arises, What are the 

 conditions necessary to accomplish its ascent and descent, so that the animal should 

 possess the will and power of rendering itself specifically lighter and heavier than the 

 ambient element? Dr. Buckland's theory allows only of a change of place in the 

 adjusting fluid from the pericardial cavity into the siphuncle, and vice versa. The 

 accompanying outline, fig. 25, from Prof. 

 Owen's dissections, shows the relative 

 position of the internal organs : — a a is 

 the enveloping fleshy mantle, dissected 

 off to expose b b, the branchiae, floating 

 in c, the branchial chamber for the re- 

 ception of the water; d is the heart, 

 with its large vascular canals surrounded 

 by clusters of glandular follicles, e e ; 

 the capacious pericardium, //, is laid 

 open to show its boundary and relation 

 to the central organs of the circulation ; 

 it is partially divided internally by 

 thin muscular septa, g g. From the 

 posterior wall of the musculo-membra- 

 nous bag there proceeds the siphuncle, 

 s s, destined to traverse all the cham- 

 bers of the shell, and the arrow shows 

 the direction of this aquiferous tube. 

 Anteriorly the pericardium communicates 

 with the branchial chamber, c, by two 

 apertures, k h, through each of which a bristle is passed to indicate the channels of com- 

 munication. From this arrangement it is evident that the pericardial bag has three 

 openings, one behind, which conducts the water into the siphon, and two before, which 

 open into the branchial chamber, into which sea water is constantly flowing to bathe the 

 respiratory organs, so that water alone is the ballast by which the Nautilus is retained at 

 the bottom, and its ejection the means by which it lightens its shell and is enabled to 

 rise to the surface at pleasure. Thus, by relaxing the anterior orifices, h h, that com- 

 municate with the branchial chamber, the water will flow into the pericardial sac, and 

 thence into the siphuncle, s. During this distension the animal and shell will be specifi- 

 cally heavier, and the Nautilus, without any other muscular efi'ort, remains at the bottom, 

 whilst by emptying the tube it becomes specifically lighter and floats up to the siirface. 



The body of the Nautilus is firmly attached to the shell by two lateral muscles which 

 are inserted into the walls of the last or dwelling chamber. 



Fig. 25, — Animal of the Nautilus pompilius, dissected by Prof. 

 Owen to show the manner in which the siphuncle terminates 

 in tlie pericardium, and how the pericardium opens by two 

 orifices into the branchial chamber. 



