92 



INVERTEBRATE ANIMALS. 



and the polype-bearing fringes are considerably longer, giving the 

 whole organism very much the appearance of a feather. In Vere- 

 tilluni (fig. 53) the upper part of the colony is short and club-shaped, 

 and bears the polypes all round its circumference. 



Another family of the Alcyonaria is represented by the so-called 

 " Organ-pipe corals," of which Tnhipora musica is a well-known ex- 

 ample (fig. 55). In this there is a well-developed sclerodermic coral 

 consisting of numerous cylindrical tubes, which are not divided by 

 vertical partitions (septa), but which are connected by strong trans- 

 verse plates. The coral is bright red in colour, and the polypes are 

 usually bright green. In its minute structure, the corallum is com- 

 posed of microscopic spicules fused together. 



Fig. 55.— A, Portion of tlie corallum of Tuhl-pora mvsica, of the natural size, showing 

 tlie tubular corallites and tlieir connecting floors ; 13, Polype of the same, greatly 

 enlarged, showing the mouth and tentacles. 



The best known group, however, of the Alcyonaria is that of the 

 (Idi-j/nindce, represented by the Sea-shrubs, Fan-corals, and the Red 

 Coral of commerce. A few of the members of this family are British, 

 but they attain their maximum in point of size and numbers in the 

 seas of the tropics. In all the (Jorgonidce the organism consists of a 

 composite structure made up of numerous polypes united by a com- 

 mon flesh or ccenosarc (fig. 56, B), the whole supported by a central 

 branched axis or coral. The coral varies in composition, being some- 

 times calcareous — as in Red Coral — sometimes horny, and sometimes 

 IJartly horny and partly calcareous, as in Isis. In all cases, however, 

 the corallum differs altogether from the sclerodermic corallum, which 

 has been described as so characteristic of the reef-building Corals. 

 The coral in the present instance is always what is called " sclero- 

 basic " — that is to say, it always forms an intei'nal axis, covered by 

 the cn'uosarc A\ith the poly])es produced therefrom. It is therefore 



