ACTINLH AND OTHER ACTINOID POLYPS. 29 
and also of the opposite or posterior one, and much less 
rapidly, if at all, along the sides intermediate. ‘This chief: 
tentacle 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 from early Paleozoic time. 
The way leading out from the Radiate structure is thus 
manifested by these tlower-like polyps. In fact perfect circu- 
lar series in organs or parts do not belong to any living organ- 
ism, 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 dis- 
guised spirals. 
The walls of the body contain two sets of muscles, a circu- 
lar and a longitudinal, the latter becoming radial in the disk 
and base. Similar muscles exist also in the tentacles, and cor- 
responding muscles in the fleshy partitions or septa of the in- 
ternal cavity. 
By means of these muscles an Actinia, whenever disturbed, 
contracts at once its body; and most species make of them- 
selves 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 ex- 
pansion depends on an injection of the structure with salt wa- 
ter, 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. 
