SQUIDS, ETC. 71 
as the animal grows, the last one occupied always being 
walled up or divided off by a partition called a septum. The 
center of all the divisions is penetrated by a tube; so, 
though living in the 
last chamber, the ani- 
mal is still connected 
with the first by a long, 
delicate, fleshy pedicle 
that extends through it. 
The different air-cham- 
bers are filled with gas, 
and by them the spe- 
cific gravity of the ani- 
mal may be increased 
or diminished. Beneath 
the mouth is a siphon 
through which water is 
ejected, thus forcing 
them along. On the 
Fic. 75.—Section of Mautilus pompilius, 7 
showing the chambers and connecting bottom they crawl with 
tube containing the fleshy pedicle. the shell upward. They 
have zo ink-bag, and 
in the female the tentacles or arms number ninety-four. 
The great fossil Ammonites, three feet across, are extinct 
relatives of the nautilus. 
VALUE.—Shell in ornamental work. 
NotEe.—The eye of the nautilus is remarkable in having no diop- 
tric apparatus, being merely an elevation bearing a minute hole that 
leads into the globe of the eye, which during life is filled with sea- 
water, and thus, according to Hensen, in place of a refracting lense 
and cornea, there is an arrangement for forming an image on the prin- 
ciple of the pin-hole camera. 
Order II. Two-gilled Cephalopods (Diéranchiata) ; 
Spirula (Spirulid@).—These small Cephalopods resemble 
squids, but contain within their bodies a delicate cham- 
bered pearly shell with separate whorls, the various rooms 
