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MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



with those of the closely allied polyps of Madrepora; enlarged polyps are frequently found on 

 colonies of Madrepora, provided with two oral apertures, and twenty-four mesenteries are 

 already present at one stage or another of their development. 



For the most part, the enlarged polyps of Porites are circular in section, and any increase in 

 the number of tentacles merely results in the enlargement of the polyp, without altering its out- 

 line. The polyp with two oral apertures had, however, assumed an oval form, but the tentacles 

 remained disposed in a single cycle, as in ordinary polyps. 



In a former paper (Johns Hopkins Circulars, June, 1900), and again on p. 416, it is shown 

 that in Porites the increase of mesenteries beyond the protocnemic stage takes place axially. by 

 the addition of bilateral pairs within either of the directive entocceles, and that in the end they 

 constitute both isocnemic and anisocnemic pairs. In some polyps the new mesenteries are added 

 within the dorsal entoccele, and in others within the ventral entoccele, but never within both 

 chambers in the same polyp. All stages from the occurrence of a single pair of new mesenteries 

 to six pairs have been already described. 



Serial transverse sections of the Porites polyp displaying the bioral condition were prepared, 

 and by studying these at different levels the arrangement of the mesenteries represented in 



--H 



■ -M 



F 



Fig. 17. 

 Pontes. — Diagram illustrating polypal fission in a bud polyp. Within the entoccele of the ventral pair of directives (III, III) six bilateral 

 pairs of mesenteries (a-J) have appeared, and the stomodreum has been divided int.' two distinct tubes. The complete and incomplete 

 pairs of mesenteries and the musculature are so arranged that when the two polypal halves separate, each polyp will have six pairs of 

 mesenteries arranged as in ordinary polyps. 



tig. 17 litis been established. Two stoniodseal tubes are distinct throughout their length, and 

 twelve pairs of mesenteries are present, six pairs associated with each stomodseum and arranged 

 as in a single polyp. All the additional mesenteries have been added within the ventral 

 entoccele, the inclosing directives (III, III) of which have been pushed widely apart. The figure 

 should be compared with fig. lie, on p. 47»), representing the arrangement in a polyp also with 

 twelve mesenterial pairs, but in which only one stomodreal tube occurs. 



From the figure itself (tig. 17), it would be difficult to determine within which of the two 

 directive entocodes the increase has taken place, or which half is primary and which half 

 secondary. In the actual sections the mesenteries on the ventral aspect are closer together, and 

 disappear first in passing from above downward. From the sections of the various polyps at 

 intermediate stages, it is clear that polypal fission in Porites is effected only after the successive 

 addition of six bilateral pairs of mesenteries, the complete and incomplete members alternating in 

 such ;i way that the moiety of the six new pairs on one side resembles the moiety of the six primary 

 on the same side. When the two fission polyps are completely separated along the axial plane, a 

 pair of directives will be formed for each polyp, by the approximation of one of the members of 

 the primary dorsal directives (IV) and the corresponding member of the last-formed ventral pair (/'). 



