THE CEPII/VLOPODA. 
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, ff g. From the 
posterior wall of the musculo-membra- 
nous bag there proceeds the siphuncle, 
8 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, /i 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 hghtens its shell and is enabled to 
rise to the surface at pleasure. Thus, by relaxing the anterior orifices, li It, that com- 
municate with the branchial chamber, the water will flow nito 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 effort, remains at the bottom, 
whilst by emptying the tube it becomes specifically lighter and floats up to the surface. 
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 dwellinof 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. 
