ACTINIAA AND OTHER ACTINOID POLYPS. rie | 
exterior skin, reticularly corrugated, occasionally becomes a 
surface of suction-warts, as in many Sagartie. 
The znternal 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; s¢z in the second; twelve in the third; 
twenty-four in the fourth; forty-erght in the fifth, and so on. 
