548 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



eight complete mesenteries ;ind four incomplete members, arranged in ])ilateral pairs, as repre- 

 sented in tig. 4. The retractor muscles on the mesogheal plaitings are clearly distinguishalile, 

 and indicate that the two axial pairs of mesenteries ai-e the directives; the complete mesenteries 

 remain attached to the stomodanim at about equal distances apart throughout its length. In 

 serial transverse sections the incomplete mesenteries usually extend below the stomodseal area, 

 but occasionally thej' cease before its lower extremitj' is reached; their vertical extent is 

 greatest in the axial polyps. As the complete mesenteries become free they all bear mesenterial 

 filaments. The four pairs, equally developed, continue after the incomplete mesenteries have 

 disappeared (lig. 5); then the dorsal directives are lost, and shorth' afterwards the ventral 

 directives, and the four lateral mesenteries continue together for some distance (fig. 6); 

 soon the dorsal of the lateral pairs disappears, and finally the last pair. Thus a definite 

 order of disappearance is followed by the six mesenterial pairs in passing a series of transverse 

 sections in review, the pair considered to be the first to appear in coral larva? extending farthest, 

 and the most recent pairs to arise extending least. Sometimes the two moities of a pair die out 

 together, but more often the mesentery on one side will continue a little below the corresponding 

 member on the other side. 



In the lower part of their course certam of the mesenteries, usually the first three develop- 

 mental pairs, become convoluted at their free edge; and as the septal invaginations here meet in 

 the middle of the polypal cavity, and give rise to distinct interseptal loculi, each mesentery is 

 frequently in a chamlier by itself. 



No indication of anj' perithccal continuation of the mesenteries occurs; in living expanded 

 polyps the partitions can be seen to pass uninterruptedly from the column wall down to the 

 calicular cavity, and serial transverse sections confirm this. 



On each face of a mesentery the endoderm immediately ))ehind the filament becomes consid- 

 erablj' swollen, and contains zooxanthellie and numerous granular gland cells; a few nematocysts 

 also are found. The endodermal enlargement extends nearl}? as far as the termination of the 

 mesenterj' (fig. 14). 



In axial polyps the portion of the mesentery peripheral to the swollen region remains long 

 and very narrow, retaining its musculature and plaited mesoghea, and zooxanthelhe occur here 

 and thei'e. The first three developmental pairs of mesenteries in radial polyps may Ijecome 

 convoluted in the lower region, each bearing a mesenterial filament of the usual tj'pe, with 

 swollen endoderm inmiediately behind (fig. (i). The filaments are here often crowded with long 

 nematocysts, somewhat recalling the acontia of Actiniie, and may persist as far as the termination 

 of the mesentery. At their origin iti the polypal wall the mesenteries are \ery narrow, and 

 are situated at about equal distances apart all round. The endodermal epithelium is a nari'ow 

 layer and without zooxanthella?. 



In the tentacular region of retracted polyps the mesentei'ics are very short transversely, and 

 the musculature is somewhat strongly developed, especially in axial polyj^s. The mesoghea is 

 here deeply plaited, but is smooth on the opposite face, where weak oblique nuiscle fibers are 

 indicated (fig. 12). In axial polyps the nuisculature of the four incomplete mesenteries is 

 developed almost as strongly as that of the eight complete mesenteries. 



The gastro-ccelomic cavity proper, independent t)f its canal outgrowths, originates in the 

 buds in one of the ordinaiy superficial canals (figs. 22-27), but later becomes much larger and 

 provided with its own sj'stem of canals, and terminates proxinially either abruptly or in canalicular 

 prolongations. 



For some distance downward the coelentei-on is approximately circular in outline, or oval 

 in many radial polyps: the contiiuiity of the walls is interrupted here and there by the canal 

 outgrowths, liut otherwise each polypal cavity is distinct. 



The septal invaginations, which are mostly entoctelic, vary in extent; usuallj- only one, the 

 axial, is very pronounced, and may almost completely subdivide the gastric cavity. Toward 

 the lowermost region, where only two or four mesenteries remain, the polypal cavity is 

 nearly always completeh' divided into two by the union of the axial and abaxial septa; in other 

 cases, as in fig. (i, the central p<M'tion is almost obliterated liy six broad triangular septal 



