508 MOLLUSCA SUB-KINGDOM VI 
several volutions, the outermost of which either envelopes all the earlier ones 
(Nautilus pompilius), or leaves the umbilicus partly open (.V. wmbilicatus). 
With the exception of the last half of the outer volution, which is occupied 
by the animal as a living chamber, the shell is divided up into numerous 
cavities or chambers by parallel partitions called septa, the mesal parts of 
which are concave toward the aperture; and they are disposed at regular 
intervals. The compartments thus formed are said by different authors to 
be filled with air, gaseous, or even fluid matter, and all are traversed by the 
siphon.! 
This organ has dense walls and is probably not capable of any extended 
movements inside of the surrounding calcareous parts which form the siph- 
uncle. The relation of the siphuncle to the septal chambers in Nautilus has 
not been sufficiently investigated to enable one to state distinctly what its 
functions may be. The whole exterior of the mantle and siphon is encased 
in a cuticle of horny matter, the remains of which are often found in the 
living chambers and siphuncles of fossil forms as well. The shell itself is 
composed of two layers, an internal and an external. The outer layer is 
composed of imbricated laminae, is porcellanous, light-coloured, and super- 
ficially ornamented with red or brown transverse bands; the inner layer is 
nacreous, and composed of thin, parallel laminae, which are crossed by fine 
rectangular lines. The septa likewise consist of a pearly layer, but are 
covered over like the inner walls of the chambers with a very thin, opaque, 
calcareous film. A large number of fossil shells have a structure similar to 
the recent Nautilus. These are divided into several groups, characterised by 
peculiarities of the initial chamber, and by differences in the suture lines, 
siphuneles, sculpturing, and form of the aperture. 
Our knowledge of the life-history of the Nautilus is very limited. 
Although empty shells are cast ashore in great quantities in the Pacific and 
Indian Oceans, the animal is rarely found alive. According to Rumphius, the 
creature swims by ejecting water through the hyponome, and at the same time 
holds the tentacles expanded horizontally, and the head protruded as far as 
possible ; but when creeping, probably the head and tentacles are directed 
downward.2 The shell is essentially alike in both cases. However, in 
Nautilus pompilius, Willey has found that the females differ in having flatter 
and more convergent sides, the males being stouter and more gibbous, which 
is exactly contrary to the prevalent notions with regard to sex among shell- 
bearing Cephalopods. The shell is supposed to serve as a hydrostatic appar- 
atus, sinking when the animal withdraws into the living chamber, but suffi- 
1 These conditions are described by Professor Verrill in the following note :—‘ The pericardium 
of Nautilus pompilius communicates directly with the gill cavity by special pores, which are close 
to the orifices of the nephridia, but do not unite directly with latter, as in most JJol/usca. Water 
can, therefore, pass directly into the pericardium and other coelomic cavities. The cavity of the 
siphuncle appears to communicate directly with the pericardium, and hence with the gill cavity by 
means of the special pores. Thus sea-water can readily pass into or out from the chambers of the 
shell, to equalise pressure at varying depths, as in most marine JZol/usca. These chambers are 
unquestionably filled with fluid under normal conditions. But living as the animal does under 
pressure at considerable depths, the fluid in the chambers is saturated with the gases in solution. 
When the Nautilus is rapidly brought to the surface, some of the gas is liberated in consequence of 
diminished pressure, and must occupy part of the space within the chambers by forcing out some of 
the fluid. Hence the shell will float until the free gases within the chambers are absorbed or 
otherwise eliminated. There is no evidence that free gases are ever naturally present in the living 
chambers during life.” 
2 Rumphius, G. £., VAmboinische Rariteitkamer, p. 59. Amsterdam, 1705. 
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