246 RADIATES: POLYPS. 



with winding trenches, Figure 504, on each side of 

 which there is a row of tentacles. The form of the 

 Mseandrinas is generally that of a hemisphere, and 

 some of these masses are twelve feet, in diameter. 

 The true Astrseans, or Star-Corals, Figure 506, have 

 the cells in the form of concave pits, and the common 

 forms of this coral are hemispherical or dome-shaped 

 masses, some of which are twenty feet in diameter; 

 and ihe polyps themselves are often an inch in diam- 

 eter. Most of them, however, are very much smaller. 

 One beautiful little Astraean, Dana's Astrangia, has its 

 home in Long Island Sound, where it occurs in little 

 clusters upon the stones and shells, from just below 

 low-water mark even down to ten fathoms in depth. 

 It thrives well in the aquarium, and eats little mol- 

 lusks and other small animals with a good relish. In 

 those coral polyps called Oculinas, the coral when 

 young spreads so as to form a broad base ; later beau- 

 tiful tufts and tree-like branches arise from this base. 

 A portion of one of these is shown in Figure 508. 



In the great group of Fungus Corals, the coral is 

 broad and flat, looking like a toad-stool without a 

 stem, as in Figure 509. Polyps of this kind have 

 short lobe-like tentacles in multiples of six. Each 

 specimen, like Figure 509, is the secretion of a single 

 polyp, and similar specimens are sometimes a foot or 

 more in diameter. 



But some of the most interesting facts about coral 

 polyps remain to be told. Hundreds of the islands and 

 .reefs in the ocean are made of coral, the skeletons 

 of Polyps. These islands and reefs are most abundant 

 and most extensive in the Pacific and Indian Oceans, 



