CHAMBERS OF NAITILVS. 321 
of chambers that are separated from each other 
by transverse plates ; these plates are perforated 
to admit the passage of a membranous tube or 
siph uncle either through their centre, or near their 
internal margin. (PI. 1. Fig. 31. PI. 32. Fig. 2. 
and PL 33.) 
The external open chamber is very large, and 
forms the receptacle of the body of the animal. 
The internal close chambers contain only air, and 
have no communication with the outer chamber, 
excepting by one small aperture in each plate 
for the passage of a membranous tube, which des- 
cends through the entire series of plates to the 
innermost extremity of the shell, (PI. 31, y. y. a. 
b. c, d. e. and PI. 32, a. b. d. e. f.). These air 
chambers are destined to counterbalance the 
weight of the shell, and thereby to render the 
body and shell together so nearly of the weight 
of water, that the difference arising from the 
siphuncle being either empty, or filled with a fluid, 
may cause the animal to swim or sink.* 
As neither the siphuncle, nor the external 
* 
The siphuncle represented in PL 31, Fig, 1, illustrates the 
structure and uses of that organ ; in the smallest whorls, from 
d. inwards, it is enclosed by a thin and almost pulverulent calca- 
\reous 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 the 
same expansion or contraction, as the membranous tube enclosed 
within it. In the fossil Nautili, a similar calcareous sheath is 
often preserved, as in PI. 32, Figs. 2, 3, and PI, 33, forming a 
connected series of tubes of carbonate of lime, closely fitted 
to the collar of each transverse plate. In four chambers of the 
recent shell (PI. 31, Fig. 1, a. b. c. d.) this sheath is partially 
c;f,ol. y 
