COELENTERATA 71 



Neither the, septa nor the eostae correspond with the 

 mesenteries, but are situated in intermesenteric spaces. Like 

 the mesenteries, they may increase in number, secondary 

 septa arising between the primary. Most of the corals which 

 form massive skeletons increase the number of individuals in 

 the colony by budding ; in some the theca of each member of 

 the colony remains distinct {Madr&pora, Oculina, Astraea) 

 (Fig. 49), in others complications arise by the fusion and 

 obliteration of the walls of the theca, etc. {Meandrina). 



Order 2. Octaotinia. 



Chaeacteristics. — Colonial AdiTiozoa with eight pinnate ten- 

 tacles and eight mesenteries, which bear the longitudinal 

 muscles on their ventral surface, that is, on the face which 

 looks towards the single siphonoglyph. , 



The well-known skeleton of the organ-pipe coral, Tubipora, 

 consists of a stolon or encrusting lamina which attaches the 

 colonies to some foreign body, and of a series of tubes in which 

 the polyps live, termed corallites, which arise from the stolon. 

 The corallites are externally connected together by horizontal 

 plates, forming the platforms or exothecal tabulae (Fig. 50) ; and 

 within each coralHte is a series of tabulae, the top one of which 



Fig. 50. — A portion of the oorallum of Tviipora 

 musica of the natural size, showing the tuhular 

 corallites and the exothecal tabulae or plat- 

 forms. 



cuts off the polyp from the dead skeleton below. New coral- 

 lites constantly arise from the platforms, so that the whole coral 

 increases in circumference as well as in height as it gets older. 

 The platforms are formed by outgrowths of the lips of the 

 hving coral, and are at first very thin; they are traversed 

 by many branching canals. 



