MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 455) 



A wide distinction in their manner of appearance thus separates the members of the second 

 order of mesenteries from those of the first, order. The primary mesenteries appear in bilateral 

 pairs, in a succession which is first toward one aspect and then toward the other aspect of 

 the polyp, and so on, and only later do they constitute unilateral pairs, in which the musculature 

 is on the faces turned toward each other. With the exception of the directive pairs, the two 

 members of each unilateral pair arise at different times, the dorso-lateral pairs being constituted 

 of mesenteries II and V. and the ventro lateral pairs of mesenteries I and VI, in the protocnemic 

 sequence; and for a long period the lateral pairs are anisocnemic. The secondary mesenteries also 

 arise in a bilateral manner, but are in unilateral (isocnemic) pairs from f/n beginning, and in any 

 polyp they are formed in only one succession, from the dorsal to the ventral aspect, alternating 

 with the primary pairs, and situated within the primary exocceles. In mature polyps the secondary 

 mesenteries are all equal, except perhaps in their vertical extent, and are arranged around the 

 polypal wall with perfect hexameral radial symmetry, all traces of their bilateral succession 

 being lost. 



Where coral polyps attain considerable size, as in Orbicella cavernosa and Phyllangia 

 amerieana, the members of the second order of mesenteries often become united with the 

 stomodseum. In doing so they follow- the same antero-posterior succession as that characteristic 

 of their order of appearance in the young polyp (see tig. 9/t, p. 464). 



SECOND CYCLE OF METACNEMES. 



The order of appearance of the second cycle of metacnemes, or third order of mesenteries, 

 may now be considered. These, when complete, consist of twelve equal pairs, a pair within 

 each of the exocceles between the protocnemes and the first-cycle metacnemes. The succession 

 has not been followed upon any coral polyp reared directly from the larva, but sufficient evidence 

 is forthcoming from the asexually produced polyps of several species to indicate in a general 

 way the manner in which it is carried out. 



Mature polyps of Gladocora arbusoula and Astrangia solitaria usually contain a certain num- 

 ber of second-order metacnemes, but apparently never the full complement of twelve pairs. It 

 is therefore possible to obtain from these certain intermediate stages in the establishment of the 

 cycle. PI. VI, fig. 49, represents a section through the stomoda?al region of a polyp of Gladocora 

 with sixteen pairs of mesenteries. Of these the six complete pairs are protocnemes, the six alter- 

 nating pairs are first-cycle metacnemes, and the four pairs remaining are the only representa- 

 tives of the second-cycle metacnemes. The latter are but feebly developed, and without 

 mesenterial filaments. The fact of greatest importance, in connection with the four new pairs 

 of mesenteries, is their restriction to only T one exoccele within each of the six primary systems; 

 they are not developed in both the exocceles within the two ventral systems, as considerations of 

 symmetry would suggest. It will be also observed that in each case they occur within the 

 exoccele on the dorsal aspect of the second-order mesenteries (cf\ p. 458). 



The polyps of Gladocora arbusoula very rarely pass beyond the stage with sixteen mesen- 

 teries, which corresponds externally with thirty-two tentacles. For the further mesenterial 

 sequence therefore other species will be employed. 



All the members of an isolated group of eight separate polyps of Astrangia solitaria were 

 decalcified and sectionized, and the stage reached in the mesenterial development of each is 

 diagrammafacally represented in ties, s (a-g). Camera lucida drawings of a transverse section 

 from two different individuals are also given on Pis. V and VI, figs. 43, 47. The seven 

 diagrammatic figures reveal that no two polyps in the group were alike in their mesenterial 

 arrangement, so that the series may be taken as affording a fairly complete representation of 

 the order of mesenterial development generally followed in this specie-. 



Fie-. s„ is taken from the smallest of the polyps. In this instance three members of the 

 protocnemes are still incomplete, and only four pairs of metacnemes have yet appeared, situated 

 within the dorsal and the middle primary exocceles. The sequence of the first-cycle metacnemes 

 is evidently similar to that of the polyps of Siderastrsea above described (p. 456). In the next 



