‘g 
21 
its shell forward it manufactured a new partition wall behind, thus 
making an additional air-cell. 
The most curious, as it has been the most puzzling part of the 
whole structure is this narrow tube which passes through the 
centre of each partition back to the very beginning. It is nearly 
all decayed now, but it was a membranous tube known as the 
Siphuncle ; it passes through a little collar formed in each wall, and 
so was strengthened and supported. We find remains of the same 
organ in fossil specimens as shown on the diagram. What is its 
function? And why were the air chambers constructed at all ? 
At first sight they would seem an incumbrance rather than a con- 
venience to the animal. I do not know that these questions have 
even yet been settled to the satisfaction of Zoologists. We can 
easily see that the air chambers would reduce the specific gravity 
of the whole shell and make it lighter for the animal to move with 
in the water, though they might be a hindrance to it in its descent 
and in its movements at the bottom. Some have imagined them 
filled and emptied with water as occasion required, others that 
nothing but air, or some kind of gas secreted by the animal, was in 
them. The clearest and most satisfactory account seems to me to 
be that given by Dr. Buckland in his Bridgewater Treatise. The 
compartments are true air chambers, completely isolated from each 
other, from the body of the nautilus, and from the water, therefore 
they cannot be alternately filled or emptied with anything. The 
siphuncle is the organ by means of which the specific gravity of 
the shell is altered. This tube is organically connected with the 
animal, commencing in a cavity surrounding the heart called the 
pericardial cavity. This contains a quantity of fluid which can 
at will be forced into the siphuncle along its whole length, thus 
leaving the pericardium empty or nearly so, and by so much, in- 
creasing the weight of the chamber portion of the shell ; or it may 
be again withdrawn from the tube so as to make it easier for the 
animal to ascend to the surface, The divisions assist in strengthen- 
ing the walls of the chambers against the pressure of the waters, 
on thesame principle that the walls of ships sailing on Arctic voyage 
- are strengthened by cross beams against ice pressure. 
Thus we see;here, as everywhere else throughout nature, an 
4 adaptation of the mechanism to the surrounding circumstances— 
which loses none of its beauty, none of its wonder, whether we 
look upon it as designed by a direct creation, or as the result of a 
Jong evolution from a more primitive form. 
A fossil form of nautilus is represented on the diagram, from 
which we gather that the ancient nautilus in all respects resembled 
its modern descendant. The same air chambers, the same siphuncle 
are preserved. The specimen figured illustrates in an admirable 
