MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 447 



the second carries with it a developmental significance. Thus the members of the firsl order 

 always appear before those of the second, the second before the third, and so on. Should the 

 first cycle consist of twelve mesenterial pairs, as is sometimes the ease, it represents the first and 

 second orders. The members of the tirst order are known as primaries, the members of the 

 second order as secondaries, the members of the third order as tertiaries, and so on. 



In the genera above mentioned the lirst order or cycle of six pairs (protocnemes) includes 

 the same mesenteries as those present in Porites and Madrepora; but usually all the pairs are 

 complete and equal, and in each ease they include two pairs of directives -sulcar and sulcular, 

 which are bilateral, and situated at opposite extremities of the polyp; the other four protocnemic 

 pairs, notwithstanding their origin as bilateral pairs, are now regarded as four unilateral pairs, 

 two on each side, the retractor muscles of each pair being on the faces turned toward one another. 



The second order of mesenteries in adult polyps also consists of six equal unilateral pairs, 

 alternating with the pairs of the tirst order, and situated within their exocielic chambers. In 

 some cases the pairs of the second order may become complete throughout the whole, or part of 

 the extent of the stomodseum, as in the large polyps of Orbicdla cavernosa; or some of the pairs 

 of the cycle may be complete and the others remain incomplete, as in the polyp of Phyllangia, 

 represented on p. 464. 



When fully developed, the third order of mesenteries comprises twelve unilateral pairs, 

 within the exocoeles formed by the pairs of the first and second orders. The fourth order of 

 mesenteries would contain twenty-four pairs, the fifth forty-eight, and so on, the mesenteries 

 of the newer cycles always occurring in unilateral pairs within the exoccelic chambers of the 

 previous pairs. The fourth-order mesenteries, however, never appear in any of the species here 

 studied, except in 'Phyllangia, where occasionally a few members may occur (p. 465). Very 

 often the mesenteries present in any mature polyp may be such as to leave the last cycle without 

 the full number of pairs necessary to complete the hexameral sequence. 



Although in the adult polyp the metacnemic pairs belonging to any cycle are approximately 

 equal in size, it by no means follows that they were simultaneously developed, any more than in 

 the case of the pairs of protocnemes; indeed, all the evidence from young polyps goes to show 

 that the mesenteries arise successively. The order of appearance of the metacnemes is fully 

 referred to on p. 455. - /. s< q. 



The mature polyps of the species belonging to the genera enumerated contain, within narrow 

 limits, a definite number of mesenterial pairs, which is characteristic of the species. As above 

 mentioned, this number may or may not complete the hexameral multiple, so that the last cycle 

 commenced may not he continued all the way round. 



Increase in number does not continue indefinitely. Orbicdla <i<Ttij»ir<i has usually twelve 

 pairs of mesenteries, six complete pairs of protocnemes, and six alternating incomplete 

 metacnemes; O. r<idi,it<i contains twenty-four pairs, the twelve pairs of the first and second 

 orders complete and otherwise equally developed, and the twelve pairs of the third order 

 incomplete (tig. 1. p. 423); the conditions are the same in 0. cavernosa. The mesenteries of 

 Solenastrsea and Oculina are. like those of 0. acropora, usually twenty-four in number, six 

 pairs complete and six alternating pairs incomplete (PI. XI. tig. 81). 



Astrangia solitaria exhibits six pairs of complete mesenteries, and within each primary 

 exocoele of adult polyps a pair of incomplete mesenteries always occurs, and in some instances 

 two or three pairs (p. 463). In this latter case one of the pairs is slightly larger than the other 

 and belongs to the second cycle of six pairs, while the smaller pair represents all that is yet 

 developed of the third cycle; hut in no instance has the full complement of twelve pairs consti- 

 tuting the third cycle occurred. In the closely allied species. Phyllangia aim ricana, the number 

 of mese .teries is always greater; hut here also the incompletion of the final cycle of twelve 

 pairs is very general. In one poly]) seetionized transversely (p. 464). ten pairs of mesenteries 

 reach the stomodanmi. These consist of the six protocnemic pairs and four of the first-cycle 

 metacnemes, the remaining two of this cycle not having yet reached the stomodseum. In the 

 uppermost stomoda?al region, however, one of the pairs becomes complete. An alternating cycle 

 of twelve incomplete pairs occurs, but one or more pairs may be rudimentary or absent. 



