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 (e. g., HJcMnopora refiexd) are hollow. The 



THECOCYATHUS CTLINDKACEUS ; FLABELLXJM PAVONINUM. 



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 



