594 LECTURE XXIII. 



of ejecting such fluid into the siphon, and thereby of compressing the 

 gas of the chambers and increasing the specific gravity of the shell. 

 Such, indeed, were the siphon dilatable, would be the natural effect 

 of the contraction of the animal within its shell. The consequent 

 pressure of the dense anterior muscular segment upon the soft 

 and yielding visceral cavity, resisted at the same time by the 

 unyielding posterior plate at every point, save that from which the 

 siphon was continued, would propel into that tube as much fluid as it 

 might be capable of receiving. These consequences would follow 

 the instinctive retraction of the animal when alarmed ; and if they 

 should take place while it was floating on the surface, it would im- 

 mediately begin to descend. If, desiring to rise again from the 

 bottom, the Nautilus should protrude its body from the mouth of the 

 shell, extend the folds of its mantle, and spread abroad its tentacula, 

 it would withdraw the pressure from the abdomen, and, by the act of 

 advancing, create a tendency to a vacuum in the posterior part of 

 the shell. The fluid in the siphon would then, if that tube were con- 

 tractile, return into the pericardial cavity, the gas in the chambers 

 would expand, and the specific gravity of the animal be sufficiently 

 diminished to cause the commencement of its ascent, which would 

 doubtless be accelerated by the reaction of the respiratory currents 

 upon the water below. 



Neither the contents nor the vital properties of the siphon are 

 however yet known : an artery and vein are assigned for its life and 

 nutrition, and to extend a low degree of the same influence to the 

 surrounding shell : but the structure of the membranous siphon, 

 in the specimens from which I have had the opportunity of exa- 

 mining it in a recent state, presents, beyond the first chamber, an 

 inextensible and almost friable texture, apparently unsusceptible of 

 dilatation and contraction : it is also coated beyond the extremity of 

 the short testaceous siphon with a thin calcareous deposit. A graver 

 objection to the hydrostatic action of the siphon is founded upon its 

 structure in certain extinct species of Nautilus, as the N. Sipho, in 

 which it is provided through its whole extent with an inflexible outer 

 calcareous tube, rendering it physically impossible that the gas of the 

 chambers could be affected by any difference in the quantity of fluid 

 contained in the siphon. In the Nautilus striatus, also, the calcareous 

 siphon is a continous tube, slightly dilated in each chamber. 



From these facts I incline to the conclusion, that the sole function 

 of the air-chambers is that of the balloon, and that the power which 

 the animal enjoys of altering at will its specific gravity, must be 

 analogous to that possessed by the fresh-water testaceous Gastropods, 

 ar»d that it depends chiefly upon changes in the extent of the surface 



