508 



MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



The phenomena presented by the early divisions of Man icina clearly prove that fission actually 

 takes place in a plane at right angles to the long- axis of the mouth and stomodseum; otherwise 

 the regular distribution to each daughter stomoda?um of six complete pairs of mesenteries, 

 derived from the primary twelve pairs, with one pair of directives only at opposite extremities, 

 would be inconceivable. Although among the many living colonies which have been examined, 

 examples in which the oral aperture or stomodanim was in the actual process of division have 

 not been observed, yet frequently two small mouths are found in close proximity, suggesting 

 that they have arisen from the splitting of a single larger aperture. 



The later divisions in Manicina reveal that the fission of the stomodanim, along with its 

 associated mesenteries, is not always median, or results in the production of equal halves. 

 Sometimes in living polyps a very small aperture will be found, as if cut off from a larger, and 

 only a few mesenteries are associated with it compared with the number united with the latter. 



FISSION IN FAVIA. 



Fa/via fragum occurs in some abundance on the reefs throughout the West Indies, forming- 

 small, convex, hemispheroidal or irregular colonies, usually four to five centimeters in diameter. 



m 



m 



m 



m 



Fig. 15 {a-d). 

 Favia fragum. — Figs. 15. Diagrammatic figures illustrating the mesenterial sequence and fission in larvae, a, Larva with three pairs of 

 protoenemes, of which only one pair is complete I cf. PI. XIV, fig. 112). b. Larva with three jirotocnemic pairs, of which two are 

 complete (cf. PL XV, fig. 113). c, Larva with rive pairs of protocnemes, of which three pairs are complete and two pairs incomplete 

 (cj. Ph XV, fig. 115). d, Larva at stage of fixation, with Ed.ward.sian mesenteries complete and fifth and sixth pairs incomplete. 



New polyps are added to the colony by division of the older polyps, apparently never by bud- 

 ding. A polyp sometimes exhibits two or three oral apertures on a single elongated or triangu- 



