506 



MEMOIRS OF THP: NATIONAL ACADEMY OF SCIENCES. 



Were the sepunitioii of tlu^ disk iind colunin wall to be completed at thi.s stag'e. across the plane 

 of tission, it is clear that two similar daughter polyps would be produced. In each ca.se one 

 moiety of the middle mesenterial pair of one side would form with the corresponding mesentery 

 from the other side a pair, attaclied to the side of the stomodanun opposite the directives. But 

 the longitudinal muscles of the two mesenteries in each pair would be arranged so as to face one 

 another, instead of turning from one another, as in the case of the directives; in other words, 

 the complete me.senteries of each polyp would include only one pair of directives, and five pairs' 

 in which the retractor muscles are vis-a-vis. The six pairs of complete mesenteries attached to 

 each stomodiBum would then constitute a tii'st cycle, the six large alternating pairs a second, the 

 twelve next a third cycle, and the odd pairs would r(>present the commencement of a fourth 

 cycle. 



The almost perfect regularity in the numl)er, arrangement, and extent of development of the 

 mesenteries found in tiie above example appears to )>(> rather exceptional, for other specimens 

 of JI(/n/'ci/ia sectionized reveal man^- irregularities. Fig. l-ih represents the mesenteriid arrange- 

 ment in a second polyp, also bearing two oral apertures. The details are those presented by a 

 single section toward the lowei' termination of each stomodanun. The hexameral regularity has 

 been altogether lost, or perhaps was never reached. But here again the plane of fission passes 

 through the entoccele of two opposite pairs of complete mesenteries, at right angles to the direc- 



3 



no i4(.. 



ilaniciiui artulala. — .\iiothcr polyp witli twci oral aperturf-s. The mt'.suiiturie.s have entirely lost their hexameral cyelic regularity, and 

 of the protocnemes only the directives at opposite ends can be determined with certainty. 



tive plane, and one of the two pairs of primary directives is found at each extremity. More than 

 six pairs of complete mesenteries are united with each stomoda'um, and only in two or three 

 regions of the polj'pal wall are any members of the second and third cycles developed; the hex- 

 ameral cyclic plan will be entirely lacking in the daughter jjolyps as in the original, and two 

 or more complete mesenterial pairs may occur in succession, without any alternating incomplete 

 members. In the sections higher than the one represented other pairs of small mesenteries 

 occur, l)ut do not in anj- way assist towards the production of the hexameral regularity-. At the 

 left end of the polyp two single large mesenteries occur, without any corresponding member to 

 complete the pairs; higher sections demonstrate that one moiety has simply disappeared in 

 advance of the other. 



In a third bi-oral )K)lyp studied the alternation of tirst, second, and third C3-cles of mesen- 

 teries was a little more regular than in the last example, but was by no means perfect all round; 

 one stomodicum was provided \\ ith six pairs of complete mesenteries, while to the other eight 

 pairs were attached. 



Young polyps of 2Iun!einu with three oral apertures are not so plentiful as specimens with 

 two or four apertures. It seems not unlikely that after the tirst division into two, each stomo- 

 dteum is again divided at about the same time, and thus the stage wdth thi'ee apertures w'ould 

 rarely occur. 



