CORAL-MAKING POLYPS. 43 
septa. Each of these septa is secreted between a pair of the 
radiating fleshy partitions, or septa, of the polyp (see figure 
p. 27); and thus the radiate structure of ordinary corals is 
nothing but an expression of the internally radiate structure 
of the polyp. When alive, the top, and usually the sides, of 
the coral were concealed by the outer skin of the polyp, in- 
cluding, above, the disk and tentacles; and into the depression 
or calicle at top, descended the stomach. 
Whether these radiating septa of the coral are secreted 
from the surfaces of the fleshy septa, or from a prolongation 
inward of the membrane forming the walls of the internal 
cavity, has not been directly ascertained. The latter view is 
sustained by Prof. Verrill, on the ground that the coral 
septa contain fibres of animal tissue. The secretion does not 
always commence at the central plane of a septum, for the 
septa are sometimes hollow within, just as the surface spines 
of some species (@. g., Hchinopora reflea) are hollow. The 

THECOCYATHUS CYLINDRACEUS, Pout: ; FLABELLUM SPHENISCUS, 1Dy 
exterior surface of the corallum, that is, the part outside of 
the calicles, is often ribbed, and the ribs are ordinarily only 
an outer extension of the interior septa; so that surface 
spines are in fact but the outer margins of septa. 
The first of the preceding figures exhibits another of the , 
forms of these simple corals. It is described by Pourtales 
