CHAMBERS OF NAUTILUS. 
321 
of chambers that are separated from eacli other 
by transverse plates ; these plates are perforated 
by a tube or siphon, passing through the trans- 
verse plates, either at their centre, or towards 
their internal margin. (PI. 1. Fig. 31. PI. 32. 
Pig. 2. and PI. 33.) 
The external open chamber is very large, and 
forms the receptacle of the body of the animal. 
The internal close chambers are void, and have 
'^o communication with the outer chamber, ex- 
cepting for the passage of a membranous tube, 
'which descends through an aperture in each 
plate to the innermost extremity of the shell, 
(Pi. .31, y. y. a. b. c. d. e. and PI. 32, a. b. d. 
f-)- These air chambers are destined to coun- 
terbalance the weight of the increasing body and 
shell of the animal, and thereby to render the 
whole so nearly of the weight of water, that the 
difference arising from the membrane of the 
■'liphuncle being either empty, or filled with a 
fluid, may cause the mass to swim or sink.* 
* The siphuncle represented in PI. 31, Fig. 1, illustrates the 
structure and uses of that organ ; in the smallest whorls, from 
inwards, it is enclosed by a thin calcareous covering, or 
sheath, of so soft a nature as to be readily scraped off by the 
point of a quill ; this sheath may admit of expansion or con- 
traction, together with the membraneous tube enclosed within 
In the fossil Nautili, a similar calcareous sheath is often pre- 
served, as in PI. 32, Figs. 2, 3, and PI. 33, and forms a con- 
•tocted series of tubes of carbonate of lime, closely fitted to 
tile collar of each transverse plate. In four chambers of the 
