Morphology of the Madreporaria. Ill 



wall, and thus for a time remain as partitions uniting the two 

 stomodaeal tubes. It is only later, when each single partition 

 begins to extend radiatelj across the disk and passes down 

 the body-wall, that each strand becomes two distinct mesen- 

 teries and the mesenterial system for each polyp is normally 

 established. The early appearance of the incomplete mesen- 

 teries and their perfect freedom from the stomodajum in the 

 fission polyps are features likewise reproduced in the young 

 bud polyps. 



It has already been mentioned that the number of tentacles 

 on the larger polyps may vary from sixteen to twenty-four, 

 indicating that the passage from the twelve-tentacle stage to 

 the stage with twenty-four is not abrupt. That such a 

 succession should characterize the appearance of the tentacles, 

 though not of the mesenteries, can be easily understood. In 

 the development of corals and anemones it is found that, as a 

 rule, the number of tentacles corresponds with the number of 

 mesenterial chambers, the appearance of new tentacles following 

 very closely upon the formation of additional mesenterial 

 chambers. In the earliest stage of mesenterial addition in 

 Madrepora (fig. 4) only eighteen mesenterial chambers are 

 fully established, neglecting the two axial chambers where 

 additions are taking place j thus there would be only as many 

 tentacles. Fig. 5 possesses two additional peripheral mesen- 

 teries, hence two additional mesenterial chambers and 

 tentacles. When all the mesenteries are connected peri- 

 pherally twenty-four mesenterial chambers will be present, 

 affording the possibility of twenty-four tentacular outgrowths. 



The enquiry may now be made as to how far tlie meta- 

 enemic sequence characteristic of Porites and Madrepora 

 conforms with what is already known of the mesenterial 

 development of the Zoantharia generally. The development 

 and anatomy of most modern corals reveal that the meta- 

 cnemes are added as unilateral isocnemic pairs within the six 

 primary exocoelic chambers, as in tig. 2, and the septa follow 

 a corresponding succession. One, two, four, &c., unilateral 

 pairs may arise successively within each primary system, and 

 in the end become arranged in one, two, three or more cycles. 

 The first cycle of raetacnemes consists of six equal pairs, the 

 second of twelve, the third of twenty-four, and so on. The 

 metacnemes, unlike the protocnemes, exhibit perfect radial 

 symmetry when the members of each cycle are fully deve- 

 loped ; the presence of two pairs of directives imparts a 

 bilateral character to the six pairs of primary mesenteries. 

 Departures from the hexameral plan are not infrequent in 

 corals, but apparently no form has been described which 



9* 



