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 (JY. umbilicatus}. 

 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. 1 



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 porcellanotis, 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, 

 siphuncles, 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 Eumphius, 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 sufli- 



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 Mollusca. 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 Mollusca. These chambers are 

 imquestionably 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." 



a Rumphius, G. E., d'Amboinische Rariteitkamer, p. 59. Amsterdam, 1705. 



