468 



MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



new pair form with the adjacent members of the primary directives an isocnemic pair of directives 

 on each side, and the moieties of the next two bilateral pairs form a pair on each side, in which 

 the muscles are turned toward each other. 



Fig. 10..-. 

 Porites.—A single additional mesentery (D) has appeared without a eiirresponding member to form a pair. 



Fig. lib is the diagrammatic representation of fig. 42, PI. V, which is taken from a section of an 

 enlarged polj'p of Poritcs. The figure of the section will give some idea of the difiicultios involved 

 in unraveling the relationships of the various mesenteries to one another. It is onlv by deter- 



Poritrs.—FnuT additional bilateral jiairs are present. The pair D, D is uniteil with the stomodaum, but below this region is shorter than 



pair C, C, showing that it belongs to the mieroonemic series. 



mining the faces of the mesenteries bearing the retractor muscles, and the proportional sizes of 

 the mesenteries, that the primary and the later mesenteries can be established in their relations' 

 to one another. Comparing tig. ll^.i with fig. 42, it is seen how the pairs in the actual section 



