ASTEROIDA. ISIS IIIPPURIS ; TUBIPORA MUSICA. 525 



injury from the violent motion of the water in which it grows. 

 There is a very interesting species of this group, which con- 

 nects in a remarkable manner the stony Corallium with the 

 horny Antipathes. This is the Isis Hippuris, in which the stem 

 is composed of both these substances alternately, so as to give it 

 a jointed appearance. It is formed by the deposition of calca- 

 reous matter at intervals along the horny stem ; and in this man- 

 ner it is endowed with a considerable degree of flexibility. If 

 the axis be submitted to the action of an acid, the calcareous 

 deposit is removed ; and its structure appears uniform throughout. 

 Although its aspect \B jointed^ therefore, no real articulations exist; 

 the flexible substance, of which the intervals are alone composed, 

 being really continued through the whole. We may perhaps 

 regard this as a kind of sketch of the Vertebrated structure. 



1097. The last group of the Asteroid Polypes differs from 

 those we have hitherto considered, in several important respects. 

 In the Tubiporidce, each polype is inclosed in a distinct cylin- 

 drical tube, which is formed by a continuation of its external 

 membrane ; and there is no communication among the individual 

 members, nor anything like the gelatinous flesh or central axis of 

 other Alcyonians. In some genera the tube is membranous, or 

 somewhat horny ; but in the Tubipora, the genus from which 

 the order takes its name, it is of firm calcareous structure. Of 

 this genus only one species, the Tubipora musica, inhabiting the 

 Indian Ocean, is known. It takes its name from the regular 

 arrangement of its cylindrical tubes by each others' side ; whence 

 it is commonly termed " Organ- pipe Coral." These tubes are 

 of a dark and rich crimson ; whilst the Polypes themselve.s have 

 a bright green colour when alive ; so that the contrast is very 

 striking. The Polypes resemble those already described, in all 

 the leading particulars ; except that the living membrane, which 

 lines their interior, does not seem to extend far down the calca- 

 reous tube ; and the ova are developed, not from its folds, but 

 from filaments prolonged from the base of the stomach, which 

 hang down into the cavity. The calcareous tube is formed by 

 the solidification of the membrane, which envelopes the Polype ; 

 and this is reflected in a funnel-shape, so as to close the mouth of 



