ACTINIA AND OTIIEIl AGTINOID POLYPS. 29 



and also of the opposite or posterior one, and much less 

 rapidly, if at all, along the sides intermediate. This chief- 

 tentacle mark 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 flower-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. 



