322 CfTAMBERS CONTAINED ONLY AIR. 
shell have any kind of aperture through which 
a fluid could pass into the close chambers,* it 
follows that these chambers contain nothing 
more than air, and must consequently be ex- 
posed to great pressure when at the bottom of 
the sea. Several contrivances are therefore in- 
troduced to fortify them against this pressure. 
removed from the desiccated membranous pipe within it, which 
has assumed the condition of a black elastic substance, resem- 
bling the black continuous siphuncular pipe that is frequently 
preserved in a carbonaceous state in fossil Ammonites. 
At that part of each transverse plate, which is perforated for 
the passage of the siphuncle, (PI. 31, Fig. 1, y. y.), a portion 
of its shelly matter projects ifiwurds to about one-fourth of the 
distance across each chamber, and forms a collar, around the 
membranous pipe, thus directing its passage through the trans- 
verse plates, and also affording to it, when distended with fluid, 
a strong support at each collar. A similar projecting collar is 
seen in the transverse plate of a fossil Nautilus. (PI. 32, Fig. 2, e, 
and Fig. 3, e, i. and PI. 33.) A succession of such supports 
placed at short intervals from one another, divides this long and 
thin membranaceous tube, when distended, into a series of short 
compartments, or small oval sacs, each sac communicating with 
the adjacent sacs by a contracted aperture or neck at both its 
ends, and being firmly supported around this neck by the collar 
of each transverse plate. (See PI. 32, Figs. 2, 3, and PI. 33.) 
The strength of each sac is thus increased by the shortness of 
the distance between its two extremities, and the entire pipe, thus 
subdivided into thirty or forty distinct compartments, derives from 
every subdivision an accession of power to sustain the weight or 
pressure of any fluid that may be introduced to its interior. 
* We learn from Mr. Owen, that there was no possibility of 
the access of water to the air chambers between the exterior 
of the siphuncle and the siphonic apertures of the transverse plates, 
because the entire circumference of the mantle in which the si- 
phuncle originates, is firmly attached to the shell by a horny 
girdle, impenetrable by any fluid. — Memoir on Nautilus Pompi- 
lius, p. 47. 
