the
bottom and sides of my pool. They grow in tufts, standing upright on
the rock, and looking exactly like hard gray seaweeds, while there is
nothing to lead you to suspect that they are anything else. Yesterday
I chipped off very carefully a piece of rock with a tuft upon it, and
have kept it since in a glass globe by itself with sea-water, for the
little creatures living in this marine city require a very good supply
of healthy water and air. I have called it a "marine city," and now I
will tell you why. Take the piece in your hand and run your finger
gently up and down it; you will glide quite comfortably from the lower
to the higher part of the leaf, but when you come back you will feel
your finger catch slightly on a rough surface. Your pocket lens will
show you why this is, for if you look through it at the surface of the
leaf you will see it is not smooth, but composed of hundreds of tiny
alcoves with arched tops; and on each side of these tops stand two
short blunt spines, making four in all, pointing upwards, so as partly
to cover the alcove above. As your finger went up it glided over the
spines, but on coming back it met their points. This is all you can
see in the dead specimen; I must show you the rest by diagrams, and by
and by under the microscope.

First, then, in the living specimen which I have here, those alcoves
are not open as in the dead piece, but covered over with a transparent
skin, in which, near the top of the alcove just where the curve
begins, is a slit (_s_ 2, Fig. 10) Unfortunately, the membrane
covering this alcove is too dense for you to distinguish the parts
within. Presently, however, if you are watching a piece of this living
leaf in a flat water-cell under the microscope, you will see the slit
slowly open, and begin to turn as it were inside out, exactly like the
finger of a glove, which has been pushed in at the tip, gradually
rises up when you put your finger inside it. As this goes on, a bundle
of threads appears, at first closed like a bud, but gradually opening
out into a crown of tentacles, each one clothed with hairs. Then you
will see that the slit was not exactly a slit after all, but the round
edge where the sac was pushed in. Ah! you will say, you are now
showing me a polyp like those on the sertularian tree. Not so fast, my
friend; you have not studied what is still under the covering skin and
hidden in the living animal. I have, however, prepared a slide with
this mem