CORALS. 



705 



between the projections, and nothing is seen over them but a thin shiny 



skin. 



Fig. 12 shows us the skeletons of two solitary corals which have 

 thus built up their pedestals to a considerable 

 height. The greater number of coral polyps, 

 however, do not remain solitary, but are 

 continually producing daughter polyps, whole 

 families together forming great colonies, each 

 member of which as it grows adds its own pedes- 

 tal to the mass. All the stony corals seen in 

 our museums are but the complicated skeletons, 

 i.e., the combined pedestals of polyps whose soft, 

 jelly-like bodies have perished. In the living 

 state these hard masses were covered by the flesh 

 of the animal colony, as a thin layer of slimy 

 matter. The' individual Sea-anemone-like animals 

 lurked in the cavities, rising and expanding their 

 search of food, but drawing them back under shelter 



Fig. 12.— Thu Onp OOKAL 

 iCaryopnyllia). 



The Six-rayed 

 Corals {Hexactinia). 



tentacles when in 



whenever threatened or disturbed. 



Corals are usually grouped according to the number of divisions, compart- 

 ments, or "rays'' in their bodies, each compartment, as in the Anemone, 

 running up into the hollow axis of a tentacle. 



Six-rayed polyps have six tentacles or (approximately) some multiple of 

 six, eighteen, twenty-four, forty-eight, and even more. 



Eight-rayed polyps have eight tentacles or some multiple of eight. 



Most of the six-rayed polyps live in great colonies, their skeletons produc- 

 ing the chalky masses with which we are much more familiar than we are 

 with the living animals. One important family of the 

 six-rayed polyps, however, the Fungidte, or Mushroom 

 corals, consists of single individuals. The median slit seen 

 along the surface of the Mushroom coral (Fig. 13) indicates 

 the position of the mouth, 

 while the many jagged ridges 

 that radiate out from it and 

 give the whole the appearance 

 of an inverted mushroom are 

 the hard, stony ridges and 

 partitions over which the body 

 of the polyp fits. When the 

 polyp is expanded, all this 

 skeleton is hidden from view 

 by the beautifully coloured 

 soft body, the whole surface 

 being one mass of tentacles, often of a vivid green, tipped with white. 

 With these tentacles, each richly provided with stinging cells, the huge 

 polyp paralyses and masters its prey, using them also as weapons of defence 

 against enemies. When disturbed, the whole animal sinks down among the 

 toothed ridges, which project through without actually piercing the thm 

 skin, and may well defy any enemy to touch him. These Mushroom corals, 

 when young, are cup-like or cylindrical like other corals, and provided with 

 a stalk ; as they grow, the sides of the cup get flatter and flatter until the 

 whole becomes quite flat, or the bottom of the original cup rises even higher 

 46 



fig. 13. — A Mushroom Cokai (Fungia). 



