174 THE BOTTOM OF THE SEA. 



polypi, suggests their division into two great classes, 

 a division which has been already indicated by the 

 essential difference we have pointed out in the 

 digestive apparatus of the different species. The 

 hydras grow from the exterior — that is to say, they 

 surround themselves with a horny or stony envelope, 

 whilst they have no interior calcareous basis or 

 polypier. The corallines present a character alto- 

 gether the opposite. They have an internal polypier — 

 that is to say, the hardest parts of the polypus tree 

 are in the interior, and the living bed in the midst of 

 which are the polypi is superficial. 



One particular kind of polypus, originally from the 

 Indian Ocean, produces a very remarkable polypier. 

 It is formed of tubes in regular juxtaposition with 

 each other, and joined by transverse partitions run- 

 ing from one to the other (fig. 31). It is for this 

 reason called the TuUpora musica, or organ-pipe 

 coral. The tubes are placed next to each other, like 

 those of a mouth-organ. In the engraving it is re- 

 presented half the natural size. 



The organ-pipe coral is of a beautiful red colour. 

 The first naturalist who observed it in the Indian 

 Ocean took it for a colony of great marine worms, and 

 it is only in recent times that its true nature has been 

 fully understood. 



This polypus is not an ordinary hydra, not with- 



