The Sea-mat or Flustra (Flustra foliacea.)
1, Natural size. 2, Much magnified. s, Slit caused by drawing in of the animal a.
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. 72). 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 (a, Fig. 72), 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 yet studied what is still under the covering skin and hidden in the living animal. I have, however, prepared a slide with this membrane removed (see Fig. 73), 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 the Sertularia.
Fig. 73.
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.
Each of these little beings (a, Fig 72) living in its alcove has a mouth, throat, stomach, intestine, muscles, and nerves starting from the ganglion of nervous matter, besides all that is necessary for producing eggs and sending forth young ones. You can trace all these under the microscope (see 2, Fig. 73) as the creature lies curiously doubled up in its bed, with its body bent in a loop; the intestine i, out of which the refuse food passes, coming back close up to the slit. When it is at rest, the top of the sac in which it lies is pulled in by the retractor muscle r, and looks, as I have said, like the finger of a glove with the top pushed in. When it wishes to feed, this top is drawn out by muscles running round the sac, and the tentacles open and wave in the water (1, Fig. 73).
Look now at the alcoves, the homes of these animals; see how tiny they are and how closely they fit together. Mr. Gosse, the naturalist, has reckoned that there are 6720 alcoves in a square inch; then if you turn the leaf over you will see that there is another set, fixed back to back with these, on the other side, making in all 13,440 alcoves. Now a moderate-sized leaf of flustra measures about three square inches, taking all the rounded lobes into account, so you will see we get 40,320 as a rough estimate of the number of beings on this one leaf. But if you look at this tuft I have brought, you will find it is composed of twelve such leaves, and this after all is a very small part of the mass growing round my pool. Was I wrong, then, when I said that my miniature ocean contains as many millions of beings as there are stars in the heavens?
You will want to know how these leaves grew, and it is in this way. First a little free swimming animal, a mere living sac provided with lashes, settles down and grows into one little horny alcove, with its live creature inside, which in time sends off from it three to five buds, forming alcoves all round the top and sides of the first one, growing on to it. These again bud out, and you can thus easily understand that, in this way, in time a good-sized leaf is formed. Meanwhile the creatures also send forth new swimming cells, which settle down near to begin new leaves, and thus a tuft is formed; and long after the beings in earlier parts of the leaf have died and left their alcoves empty, those round the margin are still alive and spreading.