brane 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.]

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,