First of all I will pass round pieces of the dry leaf (1, Fig. 10), and while you are getting them I will tell you where I found the living ones.  Great masses of the Flustra, as it is called, line 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 membrane removed and there you can observe the different parts, and learn that each one of these alcoves contains a complete animal, and not merely one among many mouths, like the polyp on Sertularia.

[Illustration:  FIG. 11.  DIAGRAM OF THE ANIMAL IN THE FLUSTRA OR SEA-MAT.

1, Animal protruding. 2, Animal retracted in the sheath, sh, Covering sheath, s, Slit. t, Tentacles. m, Mouth. th, Throat, st, Stomach. i, Intestine, r, Retractor muscle, e, Egg-forming parts. g, Nerve-ganglion.]