548 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



eight complete mesenteries and four incomplete members, arranged in bilateral pairs, as repre- 

 sented in fig. 4. The retractor muscles on the mesoglcea! plaitings are clearly distinguishable, 

 and indicate that the two axial pairs of mesenteries arc the directives; the complete mesenteries 

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

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

 but occasionally they cease before its lower extremity 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 (fig. 5); then the dorsal directives are lost, and shortly 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 1 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 certain 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 polvpal cavity, and give rise to distinct interseptal loculi, each mesentery is 

 frequently in a chamber by itself. 



No indication of any perithecal continuation of the mesenteries occurs; in living expanded 

 polyps the partitions can he 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 behind the filament becomes consid- 

 erably swollen, and contains zooxanthellse and numerous granular gland cells; a few nematocysts 

 also are found. The endodermal enlargement extends nearly as far as the termination of the 

 mesentery (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 mesoglcea, and zooxanthellae occur here 

 and there. The first three developmental pairs of mesenteries in radial polyps may become 

 convoluted in the lower region, each bearing a mesenterial filament of the usual type, with 

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

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

 of the mesentery. At their origin in the polvpal wall the mesenteries are very narrow, and 

 are situated at about equal distances apart all round. The endodermal epithelium is a narrow 

 layer and without zooxanthelhe. 



In the tentacular region of retracted polyps the mesenteries are very short transversely, and 

 the musculature is somewhat strongly developed, especially in axial polyps. The mesoglcea is 

 here deeply plaited, hut is smooth on the opposite face, where weak oblique muscle filters are 

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

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



The gastro-ccelomic cavity proper, independent of its canal outgrowths, originates in the 

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

 provided with its own system of canals, and terminates proximally either abruptly or in canalicular 

 prolongations. 



For some distance downward the coelenteron is approximately circular in outline, or oval 

 in many radial polyps; the continuity of the walls is interrupted here and there by the canal 

 outgrowths, but otherwise each polypal cavity is distinct. 



The septal invaginations, which are mostly entoccelic, vary in extent; usually 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 polvpal cavity is 

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

 cases, as in tig. 6, the central portion is almost obliterated by six broad triangular septal 



