4(54 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Fig. 9/; represents a transverse section through a polyp of P. americana, in which ten pairs 

 of mesenteries are complete; six pairs represent the protocnemes, while the other four pairs 

 belong to the second order. The remaining two pairs of secondaries are still incomplete. The 

 full complement of twelve tertiary pairs is present, except for one pair in the dorsal exocoele of 

 the two ventral primary systems. Here, as before, it will be understood that the dorsal and 

 ventral aspects were not actually determinable. 



Fig. 9i, from a still larger polyp of Phyllangia, reveals twelve pairs of complete mesenteries 

 belonging to the first and second orders, and twelve alternating pairs of incomplete mesenteries 

 representing the third order. At the dorsal extremity a few pairs of mesenteries of the fourth 

 order have also appeared. Polyps of Phyllangia rarely contain more than this number of 

 mesenteries, so that it has not been possible to follow the method of growth of the fourth 

 order. All that can be asserted from rig. 9* is that the mesenteries of the fourth order begin to 



Phyllangia americana. — Order of appearance of the metacnemes continued. Other pair* of second-cycle metacnemes (III) are present, and 

 four pairs of the first-cycle metacnemes (II) are now united with the stomodseum. The succession of growth is from the dorsal to the 

 ventral aspect. 



make their appearance at one extremity of the polyp, which is probably the same as that at which 

 the members of the second and third orders first arise. 



The sections of the polyps of Favia fragum represented on page 510, and of Manicina </r< olata 

 on page 504, reveal that in these species the order of appearance of the third-cycle mesenteries 

 follows a succession closely comparable with that in Cladocora, Astrangia, and Phyllangia. 



The order in which the twelve pairs of tertiary mesenteries are developed may be thus 

 summarized: 



The members of the third order of mesenteries arise in successive isocnemic pairs, after 

 the establishment of the secondary mesenteries, within the exocoelic chambers between the 

 pairs of the first and second orders of mesenteries. In a general way, two stages of growth are 

 distinguishable: First, a single pair arises within each of the six primary systems, that is, within 

 only one of the two exocoeles, the. succession being from one aspect of the polyp to the 



