ACTINOZOA. 



711 



united by a stem. They grow upon this fleshy trunk and cover it in time, 

 " just as a trunk of a tree is covered by the 

 bark." This stem is called a canosarc, which 

 secretes the coral, or skeleton. The madre- 

 pores are the greatest producers of the coral of 

 commerce. 



" If we examine a simple coral of this 

 group," says Professor Nicholson, " we find that 

 we have to deal with an animal in all im- 

 portant respects identical with an ordinary sea Fig - 8 36.-Corai. 

 anemone, but having a more or less complicated skeleton developed in its 

 interior." This skeleton is the corallum, and it is composed, as most people 

 are aware, of calcareous matter deposited within the polype itself; in the 

 former case the development or formation is exterior to the polype. A 

 single polype will thus secrete a deposit, and a colony of them produce a 

 compound skeleton, and as they throw out buds or young polypes, the manu- 

 facture of skeletons goes on by secretion. 



The Tubiporcs are like pipes, and the coral has been termed the 

 " organ-pipe." It is formed cylindrically and joined 

 externally. As under GEOLOGY we have examined the 

 question of coral reefs, we need not here recapitulate the 

 descriptions given in that section. 



Doctor Bariel writes of these animals as follows : " By 

 far the greater part of the Zoanthoid polypes, as they grow, 

 deposit in the cellular substance of the flesh of their back an 

 immense quantity of calcareous matter which enlarges as 

 the animal increases in size, and, in fact, fills up those por- 

 tions of the substance of the animal, which by the growth 

 of new parts are no longer wanted for its nourishment, and 

 in this manner they form a hard and strong case, amongst 

 the folds of which they contract themselves so as to be pro- 

 tected from external injury, and by the same means they 

 form for themselves a permanent attachment which prevents their being 

 tossed about by every wave of the element in which they live. The stony 

 substances so formed are called corals, and their mode of formation causes 

 them exactly to represent the animal which secretes them. The upper sur- 

 face is always furnished with radiating plates, the remains of the calcareous 

 particles which are deposited in the longitudinal folds of the stomach." 



Fig. 837. Coral. 



