l62 



CORALS 



which would be described in the hmguage of popuhir natural 

 liistory as " coralline " rather than " coral." 



HoRNERA. — In the genus Hornera (Fig. 76) the principal 

 branches arc much more solid, and, owing to the abundance 

 of the calcareous secretion, the greater part of the tubular 

 zooecia are said to be " immersed," that is to say, the out- 

 lines of the tubes are not visible at the surface. The result 

 of this is that the colony as a whole has a much more " coral- 

 like " appearance than the others. The colonies are erect, 

 profusely branched, and frequently 

 fan-shaped or flabelliform. When 

 examined with a lens the little spout- 

 like collars, from which the zooids 

 protrude, are seen to be arranged on 

 one side of the branches only, and 

 thus the fan-shaped colony may be 

 said to have a proper or anterior 

 surface and a reverse or posterior 

 surface. This arrangement of the 

 zooids on one surface only of a fan- 

 shaped corallum is not confined to 

 the Polyzoa but occurs in some of 

 the Stylasterina and Madreporaria 

 that live in deep water, and may be 

 due to the tendency of the zooids as 

 they are formed to turn towards the 

 source from which the food supplies 

 come to them. In shallow sea-water, 

 where the corals are subject to the 

 ebb and flow of the tides, the food comes to them first from 

 one side and then from the other, and the zooids are usually 

 arranged on all sides of the branches, but in deep water 

 there is frequently a prevailing current in one direction and 

 the zooids become grouped on one side so as to face it. 



On the terminal branches of Hornera the outlines of the 

 zooecia are faintly indicated (Fig. 76), but the older branches 

 have a much smoother coral-like surface owing to the 

 zooecia becoming immersed by the increase of calcareous 

 deposit. 



l"iG. 76. — Hornera liche- 

 noides. Terminal branch 

 of a specimen from off the 

 Shetland Islands. View of 

 the side on which the zooecia 

 open. X 7 diams. 



