Each of these little beings (a, Fig. 10) 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. 11) 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. 11).
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 six thousand, seven hundred and twenty 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, thirteen thousand, four hundred and forty 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 forty thousand, three hundred and twenty 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 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....
If you can trace the spore-cells and urns in the seaweeds, observe the polyps in the Sertularia, and count the number of mouths on a branch of my animal fringe (Sertularia tenella); if you make acquaintance with the Thuricolla in its vase, and are fortunate enough to see one divide in two; if you learn to know some of the beautiful forms of diatoms, and can picture to yourself the life of the tiny inhabitants of the Flustra; then you will have used your microscope with some effect, and be prepared for an expedition to my pool, where we will go together some day to seek new treasures.