128 Beautiful Shells. 
knowledge might be worth something to you if you 
could breathe under water; and he might teach you 
how to swim, but not how to sail, for, in spite of 
all poetic theories, he does the former and not the 
latter. Most usually he walks about at the bottom 
of the sea on his long arms, something like the 
Cuttle-fish, feeding on the marine vegetation; the 
shell is then uppermost. If we could look inside of 
it we should see numerous little chambers or cells, 
the larger and outermost of which only are in- 
habited by the mollusk, the others being filled with 
air render the whole light and buoyant. Through 
the centre of these chambers, down to the smallest 
of them, runs a membranous tube which can be 
exhausted or filled with fluid at the pleasure of the 
animal, and the difference thus effected in the 
weight of the shell enables it to sink or swim; in 
the latter case, up it goes to the surface, and ‘ keel 
upwards from the deep,” emerges, as the poet has 
said, but once there it soon reverses its position. 
The shell becomes like a boat it is true, but its 
inhabitant neither points a sail nor plies the oar, 
but propels itself along stem foremost by a mus- 
cular action, which by alternately compressing and 
loosening a kind of siphon, throws out jets or 
gushes of water, which, by the resistance they meet 
