Morphology of the Madreporaria. 



387 



cj'clical plan, and at this stage differs in no respects from the 

 polyps which have been previously described as arising by 

 gemmation. 



Young corallites of Manicina produced directly from larval 

 polyps have been collected exhibiting the exact corresponding 

 septal stage of fig. 1, namely, twelve large septa, twelve 

 alternating medium septa, and twenty-four very small septa. 

 Exosepta are usually, if not always, wanting in Manicina, 

 and also in many other fissiparous corals. 



It is at about the stage represented in fig. 1 that partial 

 fission is introduced. One of the most regular instances of 

 this obtained is shown diagrammatically in fig. 2. It is 



Diagrammatic arrangement of the mesenteries in a bioral polyp of 

 Manicina areoluta. 



evident that a polyp at the stage of fig. 1 has become elon- 

 gated in the direction of the principal or directive axis and 

 that fission has taken place midway at right angles to this 

 plane. Two oral apertures and two stomodsea occur, distinct 

 from one another all the way. Attached to each stomodsum 

 are six pairs of mesenteries, which include members of the 

 original primary and secondary orders. Obviously only one 

 pair of directives is available for each stomodteal system. 

 Tiie plane of fission is includetf within the shorter axis of the 

 original oval-shaped oral aperture and within the entocoele of 

 a pair of mesenteries on opposite sides. 



"Were the two halves to become entirely distinct at this 



