MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



5U7 



Fig. 14c'is a diagramiiiatic representation of tlie mesenteries in a Manlcina colony with four 

 oi'al apertures. Tlie longer axis of any one stomodffiura is now in a different plane from tiiat of 

 others, and the two primary pairs of directives ai-e widely separated, but still situated at the 

 opposite extremities of the colon\'; the two middle stomodwa have no directive mesenteries 

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

 passes through two entocoeles. 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 

 pi'obabl}' to be regarded as an individual irregularity. 



The number of mesenteries connected with each stoniodreum is inconstant; the stomodasum 

 at the right extremity Vtears 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. 

 Manicina armtata.— 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 entocoelic, except tor the irregularity at the upper left hand earner. 



cases they are wholly al)sent from the exoccele ))etween two pairs of complete mesenteries; in 

 many others only one pair occurs; while in a few chamljers 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 i)laces of 

 most forwai'd 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 

 pairs 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 stomodajum. and newer pairs appetir in their exocceles. 

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



