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. 10c. 

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



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

 enlarged polyp of Porites. The figure of the section will give some idea of the difficulties involved 

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



Poritts. — Four additional bilateral pairs are present. The pair I), It is united with the si.. modicum, but below this region is shorter than 



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



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

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

 to one another. Comparing fig. 117> with fig. 42, it is seen how the pairs in the actual section 



