DEVELOPMENT OF AGARICIA. 505 



mesenterial filaments, which are already well developed when the third 

 and fourth pairs of mesenteries have only begun to show (larva A). 

 The mesenterial filaments of the third pair of mesenteries show only 

 after the six pairs of primary mesenteries have reached a compara- 

 tively advanced stage of development (Larva I). 



The bilateral symmetry of the planula is shown in the position, as 

 well as the order of development, of the six pairs of primary mesen- 

 teries. This is clearly shown in the position of pairs / and //. The 

 two mesenteries of pair / lie almost in the same straight line in cross 

 sections, while the mesenteries of pair // if produced would meet at 

 an angle of about 45 degrees (Fig. 1, D; Fig. 2, /; Fig. 4, F). Further, 

 if the transverse sections be examined, it will be noted that the length 

 of the circumference on the ventral side between the peripheral ends of 

 the mesenteries of pair / is greater than the length of the cii'cumference 

 between the peripheral ends of pair //. Both these facts may be 

 expressed by saying that the angle between the mesenteries of pair / 

 on the ventral side is greater than the angle between the mesenteries 

 of pair II on the dorsal side. 



The transverse sections show that in the older larvae the oesophagus 

 is elliptical in cross section and that the long axis of the ellipse is dorso- 

 ventral, a condition found in Manicina areolata by Wilson ('88). 

 The oesophagus is continued further aborally on the ventral than on 

 the dorsal side. 



The mesenterial filaments of mesenteries / and // are continuous 

 with the ectoderm of the oesophagus and are formed by an aboral 

 growth of this ectoderm. These filaments show the same histological 

 structure as the ectoderm of the outer surface, lacking however nettle 

 cells. The condition found in larva I seems to show that the mesen- 

 terial filaments of mesenteries /// are developed from the endoderm 

 without any connection with the ectoderm. 



In Manicina areolata Wilson ('88) has shown that the filaments 

 of the first three pairs of mesenteries are developed from the oeso- 

 phageal ectoderm. In Aulactinia McMurrich ('91) found that there 

 was no reflection of the ectoderm as described in Manicina by Wilson 

 ('88), and that the median streak (which appears before the lateral 

 streaks) of the filaments of the first three mesenteries was developed 

 from the endoderm. 



My preparations seem to show that the gastro vascular cavity in 

 Agaricia is developed by a breaking down and splitting of the endo- 

 derm and that the mesenteries, muscle cells, and the cells which will 

 form or have formed the mesenterial filaments are the agents which 

 determine its form. 



