MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



409 



correspond one by one with those in the diagrammatic plan. It is further manifest that no other 

 arrangement of the pairs than that ottered would represent the primary mesenteries with the 

 characteristics they present in ordinary polyps. In the region here figured, the dorsal directives 



Porites.—Fig. n. Increase of mesenteries continued. All the additions occur within the dorsal directive entoccele. a, Three new pairs 



(A, C) occur, all of which are united within the stomodsium. 



belong to the smaller series of mesenteries as well as the new bilateral pair next to them, and as 

 in the previous figure the adjacent moieties of each pair constitute a pair of directives. The 



PoriVra.— Section of another polyp, helow tlio stomodieal region. Mesenteries IV, A, on each side constitute mierodirectives, and B, C are 



uliiNocncnnc pairs (cf. PI, V, fig, 42). 



next two mesenteries on each side form a unilateral pair in which the ventral moiety is large and 

 the dorsal small. 



