POSTLARVAL DEVELOPMENT. 115 



EPITHECA. 



Along with the first formation of the radiating septal upgrowths 

 appeared a narrow, peripheral calcareous ring, somewhat less opaque than 

 the septa, and for a long time wholly unconnected with them (plate 2). 

 The most careful examination of the living polyps proved that the annulus 

 was altogether external to the soft tissues ; while in preserved and cleared 

 specimens the polypal wall was found to pass within the inner border of the 

 skeletal deposit, not to be folded over it. The formation is undoubtedly to 

 be regarded as an epitheca, according to the definitions of this structure 

 given below. 



The epitheca increased in height along with the growth of the polyp, 

 at the same time often narrowing a little transversely. It remained 

 throughout uncovered by any polypal tissues on its outer surface, though 

 lined by the polypal wall on its inner surface (plate 9, fig. 53). Where 

 most fully developed its outer surface exhibited distinct incomplete annula- 

 tions, wrinklings, or accretion lines, as if representing separate intervals in 

 the deposition of calcareous matter (plate 4). By the time the polyps were 

 six or seven weeks old the lower region became discolored by the adherence 

 of foreign matter, such as filamentous algse and diatoms, and only the actual 

 margin was fresh and white. Opposite the septa the epithecal margin was 

 sometimes a little indented, but otherwise was of the same height all round, 

 somewhat exceeding that of the septa. Whenever, owing to uncongenial 

 conditions, the polyps shrunk from their former size, the peripheral deposit 

 remained behind, distinct and complete ; the growth of a second annulus 

 then took place at the new margin of the narrowed polyp, in such a way that 

 the new ring was wholly within the old (plate 5, figs. 25-27). 



Later, as the septa increased in radial length, some of them came in 

 contact with or were actually fused with the epithecal deposit, but the micro- 

 scopic structure of the two remained distinct. When the coralla were freed 

 from their surface of attachment the epitheca was seen to be a direct upward 

 continuation of the edge of the basal plate, and, like it, shows no centers of 

 calcification. It is made up of circular lamellae, the fibro-crystals of which 

 are arranged at right angles to the polypal wall (plate u, figs. 69, 70) ; the 

 epithecal deposit, in fact, corresponds to but one-half of a septum. 



The epitheca was found to vary greatly in the extent of its development 

 in the various polyps. In some of the most forwardly developed specimens 

 it was practically absent or represented only by a thickened marginal ring 

 (plate 4, figs. 19, 20) ; in others it appeared as a very distinct parapet, 



