470 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Fig. lie is from a transverse section through the stomodseal region of a polyp in which twenty- 

 four mesenteries are present, arranged in twelve bilateral pairs. The primary dorsal directives 

 (IV. IV) are incomplete at this level, as often happens in ordinary polyps. The unilateral paired 

 arrangement of the six new pairs of mesenteries, as regards the complete and incomplete moieties, 

 is exactly the reverse of that of the primary mesenteries. In the former, the incomplete 

 members have their musculature on faces directed ventralwards, while in the latter it is toward 

 the dorsal aspect. Four isocnemic pairs occur in which the retractor muscles are on the faces 

 turned away from one another (directives), and eight anisocnemic pairs in which the musculature 

 is on the faces turned toward each other. 



Of the many living polyps examined, none showed a stage beyond that represented in fig. lie. 

 In one or two instances where twenty-four mesenteries occurred, the stomodseum was found 

 to have undergone fission in the dorso-ventral or directive plane, and with each stomodseal tube 

 were associated six pairs of mesenteries, arranged exactly as in ordinary polyp. Of the six pairs 

 in each fission polyp, three belong to the primary series of mesenteries, and three to the later 

 formed pairs (p. 514). 



F. F F 



D 



Porife. —Six new mesenterial pairs have appeared, equaling in number ami corresponding in arrangement, only in reverse order, with the 



protocnemes. 



The results may be summarized as follows: 



1. In Porites new mesenteries beyond the primary six pairs are added at only one region, 

 which is within either the dorsal or the ventral directive entoccele. 



2. The additional mesenteries appear successively in complete or incomplete bilateral pairs, 

 the latest formed arising within the entoccele of the previously formed pair. Sometimes the 

 moiety of a pair on one side may arise a little in advance of the moiety on the other side. 



3. The longitudinal muscles on the mesenteries are so arranged that the members of the 

 first additional pair constitute with the sulcar or sulcular directives, as the case may be, two 

 isocnemic pairs, in which the musculature is on the faces turned away from one another. In the 

 succeeding bilateral pairs, the musculature is alternately on opposite faces, so that the eighth 

 and ninth bilateral pairs on each side form a unilateral pair in which the muscular faces are 

 turned toward each other, and likewise the tenth and eleventh pairs. On the twelfth bilateral 

 paii- the retractor muscles are on opposite faces, as in directives proper. 



1. Below the stomodseal region both the primary and additional pairs consist of alternately 



