ACTINIA AND OTHER ACTINOID POLYPS. 27 



exterior skin, reticularly corrugated, occasionally becomes a 

 surface of suction- warts, as in many Sagartia3. 



The internal structure of the Actinia is radiate like the ex- 

 ternal, and more profoundly and constantly so. The mouth, 

 a fleshy toothless opening in the disk, opens directly into a 

 stomach, which descends usually about a third of the way to 

 the base of the body ; its sides are closed together unless 

 it be in use. The general cavity of the body around and be- 

 low the stomach is divided radiately by fleshy partitions, or 

 septa, into narrow compartments ; the larger of these septa 



connect the stomach to the sides of the animal, and, besides 

 holding it in place, serve to pull it open or distend it for the 

 reception of food. The above figure represents in a gener- 

 al way a horizontal section of the body through the stomach, 

 and shows the position of the radiating septa and the interme- 

 diate compartments. It presents to view the fact that these 

 are in pairs, and another fact that the number of pairs of par- 

 titions in the ordinary Actinoid polyps is regularly some mul- 

 tiple of six, although other numbers occur during the succes- 

 sive developments that take place in the growth of a polyp, 

 and are occasionally persistent in the adult state. There are six 

 pairs in the first series ; six in the second ; twelve in the third ; 

 twenty-four in the fourth ; forty-eight in the fifth, and so on. 



