538 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



were the earliest in the phylogenetic history of the Madreporaria, the Cyclocnemaria appearing 

 comparatively late. 



It is, of course, uncertain as to how far later researches on similar lines will reveal other sys- 

 tems of mesenterial and septal development. Any other distinct type which may be discovered 

 will, however, merit recognition of equal value. Seeing that in the Zoanthe;e. among the Acti- 

 niaria, another wholly distinct type actually exists, the possibility should be home in mind. 

 Among the bilateral Palaeozoic corals growth occurred in a bilateral manner at other than the 

 dorsal or ventral axial regions; the septal growth in Zaphrentoid corals like Streptelasma and 

 Zaphrentis was undoubtedly unlike that in any of the forms here described. In many respects 

 it suggests such a development as would be followed in polyps with a mesenterial sequence like 

 that characteristic of the Zoanthea?." 



One very suggestive result of the recent study of corals is the demonstration that very 

 often an alteration of the septal arrangement takes place between its primary plan and that in 

 the mature calice. Thus von Koch (1889), by a complete series of transverse sections, has proved 

 that the octameral Caryophyllia rugosa is hexameral so far as the two primary cycles of 

 septa are concerned, and that it is only with the appearance of the third cycle that irregularities 

 are introduced which lead to the octameral plan characteristic of the mature corallum. 



In this connection the remarks of Count Pourtales, in Deep Sea Corals (1871). are also 

 particularly instructive. Discussing the order Rugosa he states: 



"Mr. R. Ludwig has shown (II. von Meyer's Palseontographica, Vols. X and XIV) that the 

 tetameral arrangement claimed for the Rugosa is only apparent, there being originally six 

 primary septa; but that further development in each system is asymmetrical, and that two of 

 the systems remain generally undeveloped. I had. before having knowledge of Ludwig's 

 researches, come substantially to the same conclusions by the examination of Lophophylkcm pro- 

 Uferum Edw. and II., from the carboniferous formation, a form very suitable for that study. 

 When the youngest stage of the coral is examined by cutting through the tip of the conical 

 Zophophyllum proliferum, six primary septa and six interseptal chambers are found, placed 

 symmetrically on two sides of a vertical plane, and unequally developed." 



This reference of the septa of the Rugosa or Tetracoralla to a primary hexameral system 

 receives additional support from Avhat is shown to be characteristic of the early polyps of the 

 Actiniaria and Madreporaria. The protocnemic stage of recent corals is hexameral. whatever be 

 the subsequent arrangement, and the evidence given above would seem to prove a like 

 protocnemic stage for the ancient corals; in which case all the mesenterial and septal divergences 

 characteristic of the fossil corals took place from this stage, as in living corals and anemones, 

 and the adult tetrameral symmetry is only secondary. 



The fact that in Porites and Madrepora only eight of the twelve protocnemes ever become 

 complete, and that in other polyps the union of the remaining pairs with the stoniodieum is 

 always long delayed, may perhaps be taken as suggestive of an ancestry in which the unilateral 

 pairs throughout consisted of a complete and incomplete moiety (anisocnemic), as in the Zoanthids 

 of to-day. 



The adult hexameral plan is by no means invariable among recent Madreporaria. Duncan 

 (1885, p. 7), discussing the definition of the Madreporaria Aporosa by Milne Edwards and Haime, 

 states: "Moreover the hexameral arrangement of the septa is not constant; it may be pentameral, 

 heptameral, octameral, or decameral." The recent deep-sea genera TIaplophyllum Pourtales, and 

 Cui/,,;, i Duncan, have been assigned tetameral septa, as is also the case with the Cretaceous 

 Holocystis M.-E. and II. It will lie necessary in these cases to ascertain the developmental 

 history of the corals before the exact value can be accorded their adult symmetry. 



Whether the primary plan of the mesenteries and septa of the Palseozoic corals were tetameral 

 or hexameral. it has been clearly shown that the mesenteries and septa increased in a bilateral 

 manner from two or more restricted regions. This was first emphasized by Ludwig (1862, 



""Relationships of the Rugosa (Tetracoralla) in tin- living Zoantheae." Johns Hopkins Univ. Circ, vol. 

 xxi, n... 155; also, Ann. Mag. Nat. Hist., ser. 7, vol. x, May, 1902. 



