514 



MEMOIRS OF THP: NATIONAL ACADEMY OF SCIENCES. 



with those of the closely allied polyps of Jfadnponr, eiilaro-(>d polyps are friMiuently found on 

 colonies of Madrepm'a, provided with two oral apertui-es, and twent_v-four mesenteries are 

 already pi'esent at one stage or another of their development. 



For the most part, the enlarged polyps of Pcr/'fex 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, liowever, assumed an oval form, l»ut the tentacles 

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



Ill a former paper (Johns Hopkins Circulars, June, lUOO), and again on p. 44(j, it is shown 

 that in IWHes the increase of mesenteries beyond the protocnemic stage takes place axially, l)y 

 the addition of bilateral pairs within (>itlier of the directive entocieles. and that in the end thej' 

 constitute both isocneniic and anisocnemic i)airs. In some polyps the new mesenteries are added 

 within the dorsal entocoele, and in others within the ventral entoccele, but never within lioth 

 chaml)ers in tlie same polvp. All stages from the occurrence of a single pair of new mesenteries 

 to six pairs have been already described. 



Serial transverse sections of the PoT-ifo? polyp displaying the bioral condition were prepared, 

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



---a 



---.m 



Fig. 17. 

 Pontes. — Diagram illustrating polypal ftasioii in a bud polj'p. Within the entocoele of the ventral pair of directives (III, III ) six bilateral 

 pairs of mesenteries («-/) have appeared, and the stomodfeum has been divided into 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 ordiitary polyps. 



fig. 17 has been established. Two stomodseal tubes are distinct thi'oughout their length, and 

 twelve pairs of mesenteries are present, six pairs associated with each stomoda>um ;ind tirranged 

 as in a single polyp. All the additional mesenteries htive lieen added witiiin the ventral 

 entocoele, the inclosing directives (III, III) of which lia\e been j)ushed widely tipart. The figure 

 .should be compared with fig. llf, on p. 47(', representing the arrangement in :i poly]) also with 

 twelve mesenteritd pairs, but in \vhich only one stomodical tube occurs. 



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

 directive entocceles the incretise 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 al)Ove downward. From the .sections of the various polyps at 

 intermediate stages, it is clear that polypal fission in Porites is eft'ected only after the successive 

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

 such a 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 ])lane, a 

 pair of directives will be formed for eticli polyp, ))v the approximation of one of th(> inembeis of 

 the primary dorsal directives (IV) and the coiTesponding member of the last-formed ventrtil pai r (/'). 



