lO 



CORALS AND CORAL ISLANDS. 



The warty prominences on some warty species have the 

 power of clinging by suction to a surface, and such Actiniae 

 often cover their sides thus with bits of shell or of other sub- 

 stances at hand. Where there are no warts the contracted 

 exterior skin, reticularly corrugated, occasionally becomes a 

 surface of suction-warts, as in many Sagartiae. 



The hiteriial sti'iidure oi 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 below 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 general way a horizon- 

 tal section of the body through the stomach, and shows the 

 posidon of the radiating septa and the intermediate compart- 

 ments. It presents to view the fact that these are in pairs, and 

 another fact that the number of pairs of partitions in the 

 ordinary Actinoid polyps is regularly some multiple of six, 

 although other numbers occur during the successive develop- 

 ments that take place in the growth of a polyp, and are 



