POSTLARVAL DEVELOPMENT. 93 



When it is recalled that the twelve primary mesenteries and their cham- 

 bers are fully established before the tentacles and septa begin to make their 

 appearance it can be understood how these latter organs may arise a cycle 

 at a time. In their development beyond the protocnemic stage the mesen- 

 teries, tentacles, and septa follow one another very closely, and such would 

 probably be the case were the different pairs of protocnernes closely succeeded 

 by their tentacles and septa.* The organs as a whole in the Zoantharia are 

 unquestionably to be regarded as developing in a bilateral dorso-ventral 

 order, not in a cyclic manner. Hitherto the polyps in Sideraslrea are alone 

 in the throwing back of the dorso-ventrality of the skeleton as far as the 

 exoccelic protosepta. 



The forked or bifurcated continuation of the protosepta, produced either 

 by continued peripheral growth or by the production of independent nodules, 

 has been observed both in Astr aides calycularis and in Caryophvllia cyathus, 

 in addition to the present species. Both Lacaze-Duthiers (1873, 1897) and 

 Von Koch (1882) give figures of the developing corallum which show that 

 the septa are prolonged peripherally much in the same way as in Siderastrea. 

 Von Koch at first considered that the forkings were concerned in the for- 

 mation of the theca by the union of those from adjacent septa, but in 

 Caryophyllia he found (1897) the true theca (Mauer) to arise independently. 

 In Siderastrea the bifurcations are found to be merely peripheral extensions 

 of the septa, becoming fused with the simple septum in the case of the 

 entosepta, but constituting two new septa in the case of the exosepta. They 

 in no way assist in the formation of a theca. 



SECOND CYCLE OF ENTOSEPTA AND THIRD CYCLE OF EXOSEPTA. 



Before describing the further development of the septa it will be helpful 

 to consider what are the septo-mesenterial relationships involved in passing 

 from a polyp with only two cycles of septa to one with three cycles. In the 

 first polyp only six pairs of mesenteries are present, and within the ento- 

 cceles of these are the six primary entosepta ; the alternating six members 

 of the second cycle of septa are within the exocceles, on radii midway 

 between the entosepta (plate 2, fig. 9). In the second polyp with three cycles 

 of septa two alternating cycles of mesenteries are present ; the first-cycle 

 septa are contained within the entocceles of the first mesenterial cycle, the 

 second-cycle septa are within the entocoeles of the second mesenterial cycle, 



*Where in larval actinians only four pairs of mesenteries are present when the tentacles begin to 

 develop, only eight of the latter appear, one from each of the eight mesenterial chambers, and the 

 other four necessary to complete the hexameral plan arise after the establishment of the fifth and sixth 

 pairs of protocnemes. 



