DEVELOPMENT OF AGARICIA. 509 



d. Stage with Twelve Primary Septa. 



A number of skeletons in this stage were obtained (Plate 5, bottom 

 photograph). They show six primary entosepta and six exosepta. 

 The epitheca covers the outer portion and is in the form of the base 

 of a cone. It is decorated with ridges running toward the oral end 

 of the polyp. The theca is seen joining the entosepta and exosepta. 

 The primary entosepta extend beyond the theca but do not reach the 

 epitheca. In a few of the skeletons a slight indication of the bilateral 

 symmetry described for the earlier skeletons could still be recognized. 



3. Comparison of the Development of the Skeleton in 

 Agaricia fragilis with that in other Hexacorallidae. 



In Astroides calycularis Lacaze-Duthiers ('73) found that each of 

 the six primary entosepta was formed from three centres of calcifica- 

 tion, the septa being in consequence Y-shaped with the upper part 

 of the Y toward the periphery. The two septa on the left in Plate 5 

 (top figure) are forked at their peripheral ends, although the prongs 

 of the fork are not so large as in A. calycularis. Lacaze-Duthiers 

 found that the other septa appear irregularly. The same species 

 has been studied by Koch ('82), who also finds the primary septa 

 forked. Both these authors find that the primary entosepta are 

 developed before the theca or epitheca. 



In Caryophyllia cyatlnis and C. claws Lacaze-Duthiers ('97) found 

 similar stages in the development. 



In the development of Sidcrastrea radians, studied by Duerden 

 (:04), six primary entosepta are developed as single continuous rods 

 without connection with the theca or epitheca. An outer ring, the 

 beginning of the epitheca, similar to the ring described in skeleton A, 

 surrounds the basal plate. The primary exosepta are developed in 

 dorso-ventral succession and independently of the entosepta. 



University of Wisconsin, 

 April, 1915. 



