506 Mr. Wright's Observations on Dr. Buckland's Theory 



animal to retain itself in that situation. Let us suppose that 

 it is the will of the animal to rise to the surface; by calling into 

 action the muscular layers of the pericardium, its watery con- 

 tents would be ejected through the two orifices h /i, a partial 

 vacuum would be thus produced, the remaining portion of 

 the fluid which filled the siph uncle would flow into that ca- 

 vity, and from thence be ejected from the body : it is clear, 

 therefore, that the nautilus would be thus rendered specifically 

 lighter, and would consequently ascend to the surface. When 

 it wishes to descend, it has only to admit the water through 

 the orifices h h, the siphuncular apparatus would be again dis- 

 tended, its gravity increased, and its descent to the bottom 

 accomplished*. 



This explanation of the action of the siphuncle is applicable 

 to the various modifications of structure observed in the me- 

 chanism of that tube, and avoids the many serious objections 

 which may be reasonably urged against Dr. Buckland'^ hy- 

 pothesis : 



1st, We have not sufficient evidence to support the supposi- 

 tion that the air contained in the chambers of the shell 

 undergoes compression ; on the contrary, we find that 

 the Nautilus Sypho from the tertiary strata of Dax pos- 

 sessed a calcareous siphon, which passed through the 

 entire chamber and entered the aperture in the adjoin- 

 ing plate ; and it can be demonstrated that the spirula 

 has a calcareous siphon of an analogous structure ex- 

 tended along the concave side of the shell, so that in these 

 mollusks the siphuncle is a continuous calcareous tube 

 incapable of dilatation, and consequently their ascent 

 and descent in the water was accomplished xmthout those 

 conditions on which Dr. Buckland^s hypothesis rests, i. e. 

 the dilatability of the siphon, and compression of the 

 confined air. 

 2nd, I have already shown that the nautilus is peculiarly 

 adapted for seeking its prey among the myriads of inver- 

 tebrata that crowd the bed of the sea. Now according 

 to Dr. Buckland, " When the arms and body are ex- 

 panded, the 'fluid remains in the pericardium, and the 

 siphuncle is empty, and collapsed, and surrounded by 

 the portions of air that are permanently confined within 

 each air chamber ; in this state, the specific gravity of 

 the body and shell together is such as to cause the 



• This explanation was proposed in a course of lectures on Fossil 

 Zoology, which I delivered at the Philosophical Institution of this town, 

 (Cheltenham) a report of which the Editor of the Naturalist has kindly 

 inserted in the last number of his valuable periodical. 



