MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



;>i 17 



Fig. 14'' is a diagrammatic representation of the mesenteries in a Manicina colony with four 

 oral apertures. The longer axis of any otic stomodseum is now in a different plane from that of 

 others, and the two primary pairs of directives are widely separated, hut still situated at the 

 opposite extremities of the colony; the two middle stomodsea have no directive mesenteries 

 inserted on their walls. The plane of separation of each stomodseal system with one exception 

 passes through two eutocueles. The rule holds so far as the divisions on the lower .side are 

 concerned, but on the left upper side the division plane is exoccelic, a condition which is 

 probably to lie regarded as an individual irregularity. 



The number of mesenteries connected with each stomodamm is inconstant; the stomodreum 

 at the right extremity hears twelve, the next fourteen, the third twelve, while that at the left 

 end has seventeen. The alternating incomplete pairs are still more irregular. In one or two 



Fig. 14c. 

 Ifani "" itrrolata.— Polyp with four oral apertures. The mesenteries exhibit a tendency to an arrangement in alternate complete and 

 incomplete pairs. The regions of most vigorous growth are at the opposite ends, where also the primary directives are still situated 

 The fission planes are entoccelic, except lor the irregularity at the upper left hand corner. 



cases they are wholly absent from the exoccele between two pairs of complete mesenteries; in 

 many others only one pair occurs; while in a few chambers two or three pairs, representing the 

 second and third cycles, are developed. As in the previous figure, the tendency in the older 

 regions is toward a system of alternating incomplete and complete pairs, and only at places of 

 most forward growth are the third and fourth cycles represented. The incomplete mesenteries 

 further exhibit great variation in the extent of development at one and the same level; some 

 pair- are nearly as large as the complete mesenteries, while others are rudimentary. The 

 variability is such as to indicate that when the members of the younger orders increase in size 

 they may ultimately unite with the stomoda-um. and newer pairs appear in their exocoeles. 

 Under such circumstances it is scarcely possible to determine a cyclical plan. 



