UNIVALVES. 147 
effected. The Mytilus, Pecten, Sea-snail, and innu- 
merable others, are enabled to fioat on the water, 
either by the assistance of strong muscles, or of 
bladders, which they inflate at pleasure, or else 
by adhering to such heterogeneous substances as are 
borne about by the waves; but in the Nautilus a 
different mode of structure, and a different process 
it discoverable. We observe, accordingly, that the 
testaceous coverings of this interesting shell-fish are 
univalve; the partitions arched and perforated; that it 
is divided into forty or more cells, which open one 
into another, by means of a perforation in the middle 
of each partition, and decrease in size as they ap- 
proach the centre of the cell: that, further, the 
animal resides in the largest apartment, and keeps 
up a communication with the others by means of a 
slender syphon, running spirally through the perfor- 
ations of the shell. 
But this is not the only purpose for which the 
syphon is designed. The office of this tube, inge- 
niously conjectured to be analogous to that of the 
swimming-bladder in fishes, is, consequently, essen- 
tial to the movements of the animal; for the gravity 
of the shell is so admirably counterbalanced by its 
empty apartments, that the weight of the whole 
apparatus is capable of being increased or dimi- 
nished, according as the syphon is filled with air 
L2 
