470 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Fig. lie is from a transverse section tlirough the stomodseal region of a ijolj'p 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 primarj^ 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 isocncmic 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 lieyond that represented in fig. lU: 

 In one or two instances where twentj'-four mesenteries occurred, the stomodaum was found 

 to have undergone fission in the dorso-ventral or directive plane, and with each stomodaeal 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). 



E ^ ^ 



D 



Porilf\-<. — Six new mesenterial piiirs Iiave Hi.peiire<l. equaling in number and eorrespondiiiir in arrangement, only in reverse order, with the 



protucncmes. 



The results may he summai'ized as follows: 



1. In P<>r/'/>:s new m(\senteries beyond the primai'v six pairs are added at only one region, 

 which is within either the doi'sal or the ventral directive entoccele. 



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

 the latest formed arising within the entocoele 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 sulculai' directives, as the case may be, two 

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

 succeeding ))ilaterai \)airs, tlie musculature is alternately on opposite faces, so that the eighth 

 and ninth bilateral \kuvh on each side form a unilateral ])uii- in which the muscular faces are 

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

 pair the retractor muscles are on opposite faces, as in directives ])roper. 



4. Below the stomodroal region both the primary and additional pairs consist of alternately 



