MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 451 



of the six pairs are in perfect agreement with "Wilson's results. The arrangement on the com- 

 pletion of the six pairs represented in von Koch's fig. 1 exactly corresponds with that of the 

 a<litli Madrepora and Porites, that is, four .pairs are complete and two pairs incomplete. 



My own results upon the larvse <>t' Manicina areolata, so far as they go, conform with those 

 of Wilson. Stages with from three t«> six pairs of mesenteries have been obtained, the last (PI. 

 XIX. fig. 135), exactly reproducing the conditions of Koch's figure of Caryophyllia. The 

 young polyps of Mitrvicina remained tor a week or two at the Edwardsia-sta.g&, the first cycle of 

 septa arising in the meantime. (See also p. 503.) 



The various stages secured in the course of the development of Fa/via fragum serve to sup- 

 plement the results of Wilson and von Koch on the two corals mentioned (Pis. XIII-XV, and 

 p. 508). The earliest stage (tin'. 112) is from non-extruded larvse obtained from a decalcified 

 colony. Three pairs of mesenteries are present, but only one of the pairs is yet complete, and 

 this divides the coelenteric cavity into two unequal chambers. In the larger or dorsal chamber 

 a second pair of mesenteries occurs, the members of which, although incomplete, bear rudimen- 

 tary mesenterial filaments. In the smaller ventral chamber the merest rudiments of another 

 mesenterial pair are also seen. The middle pair, as is generally the case in Actiniarian and 

 Madreporarian larva?, extends almost the whole length of the cavity, the filaments being strongly 

 developed all the way. The dorsal pair terminates some distance in advance of the aboral end, 

 while the ventral pair has a very restricted course, disappearing vertically before the inner end 

 of the stomodamm is reached, and centripetally never extending beyond the endodermal 

 layer. Clearly, from the proportional extent of their development, both radially and vertically, 

 the mesenteries have not appeared simultaneously, but represent the first, second, and third 

 bilateral pairs in the sequence. 



Sections of Famia larva? which had been extruded for six hours reveal the next stage (fig. 

 113). Two pairs of mesenteries are here united with the stomoda?um, and, by comparison with 

 the previous figure, the additional complete pair is evidently the dorsal — the second in the mesen- 

 terial sequence. The ventral pair is scarcely better developed than in the former polyp, 

 but in sections immediately below T the stomodaeum a new pair — the fourth — has appeared at the 

 other extremity of the polyp, and dorsal to the second pair. A few sections below this are found 

 the rudiments of another pair, situated between the first and second pairs (fig. 11-4). These rep- 

 resent the fifth pair in the mesenterial order and traverse only a few sections. As yet there 

 are no indications of a corresponding pair between the first and third pairs. 



Sections of another larva extruded at the same time present the conditions represented in 

 fig. 115. The first three pairs now extend as far as the stomoda?um, though the third pair ceases 

 its connection in advance of the others. The fourth pair is more strongly developed, and 

 rudiments of the fifth pair also occur at the stomoda?al level, but are stronger below, where also 

 an additional pair — the sixth, situated between first and third, is apparent (tig. 116). Incipient 

 mesenterial filaments are present on the members of the third pair, while on the second pair they 

 are fully developed, but do not extend so far as on the first pair. 



Finally, in larva 3 which had just settled, four mesenterial pairs have become complete, 

 and the fifth and sixth pairs are well developed in the upper part of the column, but remain free 

 from the stomodaeum (tig. 105). All the complete pairs are provided with mesenterial filaments. 



The series presented thus demonstrates that in regard to their proportional growth, both 

 vertically and radially, the time of union with the stomodosum, and the appearance of the 

 mesenterial filaments, a definite bilateral sequence is followed in the development of the 

 protocnemes. The result is as follows: 



(a) The first pair becomes the dorsal moiety of the ventro-lateral pair of mesenteries on each 

 side of the adult polyp. 



(h) The second pair becomes the dorsal moiety of the dorso-lateral pairs of the adult polyp. 



(c) The third pair forms the ventral directives. 



(d) The fourth pair constitutes the dorsal directives. 



(e) The fifth pair becomes the ventral moiety of the dorso-lateral pair of mesenteries on each 

 side of the polyp. 



