40 OBSERVATIONS ON THE ANIMALS 
air, and have no communication whatever with either the body of the 
animal or with the siphuncle, after they are once closed in. The 
whole shell is thus lighter than the water displaced by it, and proba- 
bly just so far counterbalances the weight of the animal as to make . 
the whole, when the siphuncle is empty and collapsed, float at the 
surface of the sea. It must be remembered that the siphuncle passes 
into the pericardium, which is a large sac filled with fluid, and con- 
taining the heart ; and as it opens freely into that cavity, and has no 
muscular fibre to contract it, must be entirely dependent for filling 
or emptying on the amount of pressure on the pericardial sac. 
When the animal is at the surface it floats with the back of the 
shell above the surface, the arms and body being expanded; and thus 
the cavity into which the siphuncle passes is filled with its fluid 
secretions, the tube itself being, as before observed, empty and col- 
lapsed. 
Now let a sudden danger be supposed to occur. At once the arms 
and body are drawn within the shell—a pressure is caused on the 
exterior of the pericardium—the fluid in it is forced into the empty 
tube ; and since there is no communication between the empty 
chambers, the whole mass is reduced by the alteration of place of this 
fluid, and the specific gravity of the whole is, of course, diminished. 
The animal sinks. 
Again, when it is crawling, by means of its long arms, along the 
bottom of the sea in search of food, the arms, it is true, must be ex- 
panded, but the body remains closely confined in the shell, which is 
the lightest part, and floats over the body without any tendency to 
fall on one side. 
The importance of the siphuncle in the animal economy of this 
class will now be apparent ; and it will be no longer a matter of sur- 
prise that so small an organ should be made use of to separate the 
Siphonifera into two families, which have been called respectively the 
Nautilacea and Ammoneata, from the respective types Nautilus and 
Ammonite. The distinction is that, in the former, the siphuncle is 
placed either in the centre or nearer the inner or ventral margin, 
while in the latter it is, without exception, on the dorsal margin. 
There are other differences resulting, probably, from this ; the chief 
of them being the greater simplicity of the septa in Nawtilacea, and 
in most cases a comparatively larger siphuncle than is found in the 
species referred to the other family. Besides, however, these two 
groups, there is a third, called by M. D’Orbigny Peristellata, which 
includes the Belemnite, a genus departing somewhat widely from any 
