MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



505 



few oral apertures, and of a size suitable for sections, can be obtained. H. V. Wilson (1888) 

 has traced the development of the primary mesenteries in the larvffi 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 hexamcrous C3^cle8. 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 early stage the polyps of 

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

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

 503, 504 represent most of the stages in the appearance of the mesenteries of Ifanicina. The 

 earlier sequences have been already descril)ed (p. 450), so that attention need tie directed only 

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

 polyps of M. areolata., with the disk bearing only one, two, or four oral apertures, are not 

 infreciuentl}' found, fixed to older colonies of the same species, or to other corals or small 

 pebbles. 



Fig. 14a. 



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

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

 with each stomodseum 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 entocoele of the middle pair of complete mesenteric^ on each side. 



Fig. 14:i( represents the conditions in a transverse section of Manicina. through the 

 stomodwal 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 dg. 13(/, and compri.se 

 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 

 oi'der, 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 stomodajum has become 

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

 with each .stomodieum. The plane of fi.ssion pa.s.ses through the entoctele of the middle lateral 

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

 extremities of the pol_yp remains attached to each stomodanini. 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. 



/ 



