252 COMPARATIVE ZOOLOGY. 



Polyps dying below as new ones are developed above. The 

 living part of an Astrcea is only half an inch thick. The 

 growth of the branching Madrepore is about three inches 

 a year. The prevailing color of the Coral Polyps is 

 green ; and the usual size varies from that of a pin's head 

 to half an inch, but the Mushroom-coral (which is a single 

 individual) may be a foot in diameter. 



Corals are of two kinds : those deposited within the tis- 

 sues of the animal (sclerodermic), and those secreted by 

 the outer surface at the foot of the Polyp (sderobasic). 

 The Polyps producing the former are Actinoid, resem- 

 bling the Actinia in structure. 131 The skeleton of a single 

 Polyp (called corallite, Fig. 95) is a copy of the animal, 

 except the stomach and tentacles, the earthy matter being 

 secreted within the outer wall and between each pair of 

 partitions. So that a corallite is a short tube with vertical 

 septa radiating towards the centre. 13 " A sclerobasic Coral 

 is a true exoskeleton, and is distinguished by being smooth 

 and solid. The Polyps, having eight fringed tentacles, are 

 situated on the outside of this as a common axis, and are con- 

 nected together by the fleshy ccenosarc covering the Coral. 



( 1 ) Sclerodermic Corals. Astrcea is a hemispherical mass 

 covered with large cells. Meandrina, or " Brain-coral," 

 is also globular ; but the mouths of the Polyps open into 

 each other, forming furrows. Fungia, or " Mushroom- 

 coral," is disk-shaped, and differs from other kinds in be- 

 ing the secretion of a single gigantic Polyp, and in not 

 being fixed. Madrepora is neatly branched, with pointed 

 extremities, each ending in a small cell about a line in 

 diameter. Porites, or " Sponge-coral," is also branching, 

 but the ends are blunt, and the surface comparatively 

 smooth. Tubipwa, or " Organ - pipe coral," consists of 

 smooth red tubes connected at intervals by cross-plates. 

 The Astrcea, Meandrina, Madrepora, and Porites are the 

 chief reef-forming Corals. They will not live in waters 



