i8o THE SCIENCE OF LIFE. 



branching. Astrcea is a hemispherical mass covered with 

 large cells. Meandrina, or " Brain-coral," has the mouths 

 of the polyps opening into each other, forming furrows. 

 (Fig. 79.) Madrepore branches, like a tree, with pointed 

 extremities. (Fig. 80.) 



Sclerobasic corals are those which secrete coral by the 

 outer layer of the inverted ectoderm. Most of these 

 are of the order Alcyonaria, whose polyps are character- 

 ized by primate or fringed tentacles in multiples of four, 

 while the sclerodermic corals generally belong to the 

 order Zoantharia, with polyps having simple tentacles in 

 multiples of five or six. The characters of Alcyonarian 

 polyps may be seen by placing in sea-water some of 

 those large yellowish, gristly masses, sometimes cast up 

 by the sea, known as " dead men's fingers." From the 



FIG. 81. i. Sea-fan. 2. Sea-pen. 



surface of each pore the tentacles round the mouth of 

 the polyps will protrude, showing their general resem- 



