38 OBSERVATIONS ON THE ANIMALS 
appear to be many well-marked natural differences between these two 
groups, although the ground of separation may not seem very clear, 
as it depends on the nature of the aperture leading from one chamber 
to another. More, however, is meant by this, than a stranger to the 
nomenclature might imagine; for it includes, in a great measure, the 
consideration of how the animal and its habitation were connected 
and dependent on each other. If, for instance, the apertures be- 
tween the chambers are in regular succession, and (as they commonly 
are) produced a little way beyond the septa or walls, forming a suc- 
cession of short tubes, we at once conclude that a tube has passed 
through all of these, connecting the body of the animal with the very 
first chamber formed. This we assume by analogy, from what we 
know of the anatomy of the Nautilus. On the other hand, if there is 
merely a set of irregular holes in the walls of separation, we see that 
there could not have been any such communication; and it follows, 
almost as a matter of course, that the shell was altogether internal, 
and useful chiefly as a float. There are other differences between 
the two orders, which will be hereafter considered. It is enough at 
present that they should appear clearly separate, and that not only by 
the shell, but, as far as we know, by the structure of the animal. 
Let us begin, then, with the Siphonifera ; and in order that the 
nature and uses of the siphuncle may be fully understood, let us trace 
the history of an individual belonging to a known genus, as Nautilus, 
from the first developement of the shell to the period of its attaining 
that complicated form which well entitles it to the name of multilocu- 
lar, or many-chambered. 
That part of a shell-bearing animal upon which the shell is formed 
is a loose, muscular coat, provided with numerous glands for the 
secretion of calcareous matter, which, on being exuded, hardens, and 
becomes, in fact, the shell. The rudiments of this defence—which, 
however complicated it may be in the full-grown individual, is always 
simple in the early part of its existence—are found in the egg; and 
there is every reason to suppose that the animal of Nautilus is, at the 
time of its first exposure to the water, covered with a cup-shaped 
shell corresponding to the form of the animal. The attachment to 
the shell is partly by two muscles at the sides, and partly, doubtless, 
by that siphuncle which we have already alluded to, and which pro- 
ceeds from the region of the heart, passes through some important 
viscera, and passes out through the mantle, to be attached to the end 
of this first chamber. This siphuncle is provided with an artery, 
and seems to retain its vitality during the life of the animal. It may 
s 
