COELENTERATA 7 
Neither the septa nor the costae 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 distinet (Madrepora, Oculina, Astraea) 
(Fig. 49), in others complications arise by the fusion and 
obliteration of the walls of the theca, ete. (Meandrina). 
Order 2. OCTACTINIA. 
CHARACTERISTICS.—Colonial Actinozoa 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, Zubipora, 
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 corallite is a series of fabulae, the top one of which 
Fie. 50.—A portion of the corallum of Tubipora 
musica of the natural size, showing the tubular 
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 
living coral, and are at first very thin; they are traversed 
by many branching canals. 
