MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



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few oral apertures, and of a size suitable for sections, can he obtained. II. V. Wilson (lsss) 

 has traced the development of the primary mesenteries in the larvae of this species, from the 

 first to the sixth pairs, and has also shown that in young polyps, provided with only one oral 

 aperture, the mesenteries are arranged in three bexamerous cycles. The first cycle comprises 

 twelve pairs of complete mesenteries, two pairs of which are directives; the second cycle also 

 contains twelve alternating pairs; and the third twenty-four. At this earlj stage the polyps of 

 Manicina therefore correspond exactly, so far as regards the mesenterial arrangement, with 

 any normal hexactinian Madreporarian or Actiniarian polyp. The diagrammatic figures on pp. 

 503, •">'»! represent most of the stages in the appearance of the mesenteries of Manicina. The 

 earlier sequences have been already described (p. 450), so that attention need he directed only 

 to the later stages, which illustrate the phenomena of fissiparity. In Kingston Harbor young 

 polyps of .)/. areolata, with the disk hearing only one, two, or four oral apertures, are not 

 infrequently found, fixed to older colonies of the same species, or to other corals or small 

 pebbles. 



Fig. 14a. 



Manicina areolata.— Figs. 14. Diagrammatic figures illustrating fission, a. Polyp with two oral apertures, twelve pairs of complete mesente- 

 ric- il. II i, twelve alternating second-cycle pairs (III), twenty-four third-cycle pairs, and a few members of a fourth cycle. Associated 

 with each stomodieum are six pairs of mesenteries, three pairs of which are protocnemes, a pair of directives being at opposite 

 extremities. The plane of fission is within the entoccele of the middle pair of complete mesenteries on each side. 



Fig. 14« represents the conditions in a transverse section of Manicina through the 

 stomodfeal region of a polyp with two oral apertures. The twelve pairs of complete mesenteries, 

 including the two pairs of directives, represent the first and second cycles of fig. 13y, and comprise 

 two alternating orders, primary and secondary, each of six mesenterial pairs; the twelve pairs 

 of large incomplete mesenteries constitute the third order, the twenty-four next in size a fourth 

 order, while here and there, at regions of most forward growth, occur rudimentary pairs, 

 which are the first indications of a fifth order. The originally simple stomodieum has become 

 divided into two, and half the complete mesenteries of the primary polyp are now associated 

 with each stomodasum. The plane of fission passes through the entoccele of the middle lateral 

 pair of complete mesenteries on each side, and a single pair of directives at the opposite 

 extremities of the polyp remains attached to each stomoda?um. The plane of fission is thus at 

 right angles to the directive plane, which is also the plane including the longer oral axis of the 

 simple polyp. 



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