44 OBSERVATIONS ON THE ANIMALS 
As many as seventy chambers have been counted in these enormous 
specimens. 
We have thus, in the family of Nautilacea, a series of genera of 
chambered shells, with siphuncles running through them, the form of 
the shell varying from that of a straight cone to a spiral, in which 
all the inner whorls are hidden by the last one ; and the question 
now to be considered is the probable amount of change in the animal 
economy which corresponded to these alterations of shape. 
Now we know, in the first place, that the Nautilus is an entirely 
external shell, capable of some range in the depth at which its inha- 
bitant lives, and therefore sufficiently strong in the structure of air- 
chambers to resist the increased pressure arising from increased 
depth in the water. What conclusions shall we, then, arrive at from 
the structure of the Endosiphonite, where the septa are stronger, in- 
asmuch as they present more points in their intersection with the 
shell, and where the pressure acts immediately upon the whole sur- 
face, and not intermediately, as in the Nautilus, where the whorls 
successively defend each other? It seems probable that this new 
genus was an external shell, sometimes rising to the surface, like 
Nautilus ; but from the narrower and less rounded appearance of the 
shell, and from the shape of the septa, we should conclude that it 
belonged to an animal of rather greater activity, and one, perhaps, 
more capable of following its prey along the muddy bottom of a sea, 
than the inhabitant of such a shell as the Nautilus. 
There seems a very great probability that the animal of Spirula 
incloses within its mantle the whole or greater portion of the shell ; 
and from the close analogy of the Lituite, doubtless, that also was 
internal. We should expect an animal thus independent to be en- 
dowed with greater powers of locomotion than one encumbered with 
a house upon its back very much larger than its body ; and probably 
all the free Cephalopods are more swimming than creeping animals. 
In support of this opinion the Spirula is known to be a very thin and 
brittle shell, and the apertures in the septa, instead of opening a sim- 
ple communication from chamber to chamber, are united by a calca- 
reous tube passing continuously from the last or outer septum into 
the first chamber. The size of the last chamber is also, in every 
known specimen, very small; and although, from the brittleness of 
the shell, the aperture might and would easily become injured, still, 
out of the number that have been seen and brought away, something 
would surely have been found to indicate this extension, if it had ever 
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