SUPPLEMENTARY NOTES. 609 
siphunciilar action here proposed, whether the fluid alternately 
admitted to and rejected from the siphuncle be derived from the 
Pericardium, or from any other source within the body, or even 
from the sea ; in the former case, we have ascertained the ex- 
istence of a mechanism whereby the movements of the pericardial 
fluid maybe effected, as in the Nautilus Pompilius ; in the latter 
cases the mechanism for adjusting the passage of the fluid to and 
from the siphuncle remains yet to be discovered. 
In the case of siphons which are surrounded by unyielding 
rigid shell throughout their whole extent, (as in the Nautilus 
Sypho,) the elasticity of the air within the chambers cannot aid 
the muscular power of the siphuncle, in regulating the action of 
any fluid within that tube ; and if the hypothesis suggested (P. 
359, Note 1. 9.) respecting this species should be inapplicable to 
it, and to other animals which have an inflexible shell around 
the siphuncle, their method of moving the fluid to and from this 
organ is yet unknown. 
In the case of jointed sheaths like those at PI. 32, Fig. 3, d, 
e,/, and PI. 33, each calcareous joint (e,) if composed of rigid 
shell, may have articulated with the collars of the adjacent trans- 
verse plates {h, i,) so as to form a moveable collar valve, of 
which the superior margin being raised a little on the outside of 
the upper collar (/i,) would leave an opening between the lower 
margin of the valve and the inside of the subjacent collar (i) ; 
through this opening air might pass from the contiguous air 
chamber into the space between the calcareous sheath and mem- 
branous siphon, as often as it was emptied of its pericardial 
fluid, and when this fluid filled the siphon, the air might return 
by the same passage into the air chamber, and the lower margin 
of the valve fall into its socket within the lower collar (i). 
It is possible that in the Spirulaand other animals that do not 
withdraw their bodies into the shell, the only function of the air 
chambers may be to counterbalance the weight of the body, and 
give it buoyancy ; in such cases the use of the siphuncle may be 
to carry down to the extremity of the shell, and send off into 
each air chamber, vessels necessary to maintain the vitality of the 
interior of the shell, and of the transverse septa. The mode of 
ascent and descent ascribed to the Nautilus Pompilius is inap- 
GEOL. R R 
