Corals and coral islands.

ACTI'NIAIX AND OT'HER ACTINOID POLYPS.

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and also of the opposite or posterior one, and much less
rapidly, if at all, along the sides intermediate. This chieftentacle marks properly the true front or anterior side of
the polyp. A fore-and-aft structure is also very strongly
marked in some of the ancient cyathophylloid corals, and
hence it belonged to the type fronm early Paleozoic time.
The way leading out from the Radiate structure is thus
manifested by these flower-like polyps. In fact perfect circular series in organs or parts do not belong to any living organism, not even to the true flower; for growth is fundamentally
spiral in its progress, and there must be always an advance
end to the spiral of growth; all apparent circles are only disguised spirals.
The walls of the body contain two sets of muscles, a circular and a longitudinal, the latter becoming radial in the disk
and base. Similar muscles exist also in the tentacles, and corresponding muscles in the fleshy partitions or septa of the internal cavity.
By means of these muscles an Actinia, whenever disturbed,
contracts at once its body; and most species make of thenselves a spheroidal or conoidal lump, showing neither disk
nor tentacles. One example of this contracted state is presented
on the frontispiece in figure 3a. After a brief period of quiet
the polyp commonly reassumes its full expansion. The expansion depends on an injection of the structure with salt water, which is taken in mainly by the mouth. As the whole body
is thus filled and injected, the flower slowly opens out, and
shows its petal-like tentacles. On contraction the water is
suddenly expelled through the mouth, and by pores in the sides
of the polyps, and at the extremity of the tentacles, and the
tentacles disappear, along with the disk, beneath the adjoining
sides of the body which are drawn or rolled in over them.