12 CORALS AND CORAL LS LANDS. 



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 mani- 

 fested *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 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 3<^. After a brief period of quiet 

 the polyp commonly reassumes its full expansion. The ex- 

 ])ansion 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. 



The Actinia appears, at first thought, to be well prepared for 

 securing its prey through its numerous tentacles. Bat these 

 are generally too short for prehension. Yet the disk often aids 

 them by rolling over the captured animal, and pushing it down 

 into the stomach. At the same time, the mouth and stomach 

 are both very extensile, so that an Actinia may swallow an 

 animal nearly as large as itself; it gradually stretches the 



