7o8 



COELENTERA TA—II. CNIDARIA. 



Fig, 17.— Oegan-Fitb CoaAL 

 (Tubipora musica). 



chalk and horn alternately. The fleshy crust which covers the axis, and out 



of which the individual polypa protrude, is 



supported by chalky spicules or plates 



scattered through its substance (see Fig. 



18). There are, however, massive forms 



produced by the eight-rayed polyps which 



have no central skeletal axis, such as the 



Blue Coral (Heliopora) and the Organ Pipe 



Coral (Fig. 17). In the latter, the polyps 



grow up side by side in separate tubes 



which result from the fusion of coloured 



chalky spicules. Those tubes, from which 



the coral takes its name, are, at regular intervals, joined together by 



platforms. It is from these platforms, where the interval between two 



tubes is considerable, that new buds arise to grow up alongside of the older 



polyps. Both the skeleton and the soft parts of this colony are of a deep 



crimson, the tentacles of the polyps being of an emerald green. 



The coral we probably know best in its skeletal condition, the Red Coral 

 of commerce, is also the product of one of the eight-rayed corals. Some 



idea of the structure of this coral in its living 

 condition may be gained from the diagram- 

 atic section of it given in Fig. 18. The 

 central hard (chalky) axis answers to the 

 smooth red branch with which we are 

 familiar. This is covered by a layer of soft 

 tubes, some of which, in the illustration, are 

 thrown back in order to reveal the hard axis, 

 the grooved markings on which show where 

 the canals ran. These tubes convey the 

 nourishing fluid through the whole colony, 

 receiving it from the polyps and giving it off 

 through branches connecting them with the 

 soft crust that lies over them. Throughout 

 this crust star or plate-like spicules are 

 scattered, which give it some degree of 

 firmness. Three polyps are seen in the 

 illustration : the one to the right has its 

 fringed tentacles extended in search of prey, 

 the middle one is in a retracted condition, and that to the left is cut across 

 to show the partitions in the body. The soft bodies of these polyps are pure 

 white, and the effect of their feathered tentacles, projecting here and there 

 from the bright red crust, is very beautiful. 



These corals form tree-like growths often several feet high, and are very 

 plentiful in the Mediterranean. Coral fisheries on an extensive scale are 

 carried on, chiefly by Italians, off the coasts of Italy, Algiers, and Spain, the 

 corals being dredged for with nets. The soft living crust is easily removed, 

 and the axis, which takes a high polish, is used for ornamental purposes. 



Though the two corals last described, the Organ Pipe Coral and the Red 

 Coral of commerce, as well as a few other of the Octactinia, are rigid 

 growths, in the majority of the eight-rayed corals, which have horny axial 

 skeletons, the branches are not hard and stiff like those of the stony corals, 

 but can wave about with the motion of the water, the brightly-coloured 



Fitj. 38. — TRk Rbi) Coral of Com- 

 MBRCB (Corallwm rubrmn). 



