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SMITHSONIAN MISCELLANEOUS COLLECTIONS 



[vol. 47 



regular and well-defined order, which is uniform for all the species 

 yet studied. The first two or three pairs arise around the oral ex- 

 tremity of the larva, while the others first appear at varying distances 

 down the wall. The protocnemic sequence is represented by the 

 Roman numerals in figure 10, and agrees with that established for 

 the greater number of actinians. The first four pairs very early unite 

 with the stomodseum, but the fifth and sixth pairs remain free or in- 

 complete for a lengthened period, suggesting a different phylogenetic 

 significance from the others. 



The six pairs of second cycle mesenteries (metacnemes) arise after 

 fixation, but in a manner altogether different from that followed by 

 the first cycle. They appear on the polypal wall in unilateral pairs 

 or couples within the six primary exocceles, and in a succession which 



Fig. 15. — Arrangement of the mesenteries in a polyp of Mankind with four 

 oral apertures and four stomodseal systems. The cyclic hexameral plan is de- 

 parted from, and only the two primary pairs of directives (d) are present. 



is from the dorsal to the ventral side of the polyp, not the whole 

 cycle at a time. For a long time, as shown in figure 11, the six 

 pairs present a difference in size, corresponding with their dorso- 

 ventral or antero-posterior order of appearance. 



The twelve pairs of third-cycle mesenteries are found to develop 

 in a succession which is altogether unexpected. They follow the 

 same dorso-ventral order as the second cycle pairs, but in two series. 

 A primary series of six pairs — one pair within each sextant — ap- 

 pears within the exoccele on the dorsal aspect of each of the second 

 cycle mesenteries, one pair following upon another, and then another 

 series of six pairs arises on the ventral aspect of the second cycle 

 mesenteries in the same order (figure 12). In the later stages of 

 growth the regularity of the mesenterial succession is not always 



