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 



