184 MANUAL OF THE MOLLUSCA. 
calculated, by their forms, for swimming; and the straight- 
shelled orthocerata and baculites must have held a nearly vertical 
position, head downwards, on account of the buoyancy of their 
shells. The use of the air-chambers is to render the whole 
animal (and shell) of nearly the same specific gravity with the 
water.* The object of the numerous partitions is not so much 
to sustain the pressure of the water, as to guard against the 
collisions to which the shell is exposed. They are most compli- 
cated in the ammonites, whose general form possesses least 
strength. The purpose of the siphuncle (as suggested by Mr. 
Searles Wood) is to maintain the vitality of the shell during the 
long life which these animals certainly enjoyed. Mr. Forbes 
has suggested that the inner course of the hamites broke off as 
the outer ones were formed. But this was not the case with the 
orthocerata, whose long straight shells were particularly exposed 
to danger; in these the preservation of the shell was provided 
for by the increased size and strength of the siphuncle, and 
its increased vascularity. In endoceras we find the siphuncle 
thickened by internal deposits, until in some of the very cylin- 
drical species it forms an almost solid axis. 
The nucleus of the shell is rather large in the nautili, and 
causes an opening to remain through the shell, until the wmbilicus 
is filled up with a callous deposit; several fossil species have 
always a hole through the centre. 
In the ammonites, the nucleus is exceedingly small, and the 
whorls compact from the first. 
It has been stated that the septa are formed periodically; but 
it must not be supposed that the shell-muscles ever become 
detached, or that the animal moves the distance of a chamber 
all at once. It is most likely that the adductors grow only in © 
front, and that a constant waste takes place behind, so that they 
are always moving onward, except when a new septum is to be 
formed; the septa indicate periodic rests. 
The consideration of this fact, that the nautilus must so 
frequently haye an air-cayity between it and its shell, is alone 
sufficient to conyince us that the chambered cephalopods could 
* A nautilus pompilius (in the cabinet of Mr. Morris) weighs 1lb., and when the 
siphuncle is secured, it floats with a 2lb. weight in its aperture. The animal would 
have displaced two pints (= 23lbs.) of water, and therefore, if it weighed 3lbs., the 
specific gravity of the animal and shell would scarcely exceed that of salt water. 
j The siphuncle and lobed septa did not hold the animal in its shell, as Von Buch 
imagined: that was secured by the shell-muscles. The complicated sutures perhaps 
indicate lobed ovaries; they occur in genera which must have produced very small 
eggs, 
