POSTLARVAL DEVELOPMENT. 121 



ever, very narrow, canal-like slits, and from these the adult gastro-ccelomic 

 cavity is derived by the disintegration of the more central endoderm and the 

 shrinkage of that lining the body-wall and mesenteries. 



Some of the larvae of Siderastrea sectionized reveal a gastro-ccelomic 

 cavity further developed than that represented in figs. 51 and 52. Below the 

 stornodseal region the parenchymatous endoderm has broken down, and the 

 middle of the cavity is occupied by organic debris, in which Zooxanthellae, 

 cell walls, and granules of various kinds are recognizable. There is no 

 doubt that this is the organic debris which is extruded from time to time by 

 the larvae soon after their liberation from the parent polyp (p. 58). At the 

 stage in the young polyp at which the septa are well advanced (plate 9, fig. 

 53), the endoderm has become a simple epithelial layer throughout, resem- 

 bling in all respects that of the mature polyps. 



In the earliest larvae sectionized eight mesenteries were already developed, 

 arranged in four bilateral pairs, as shown diagrammatically in fig. 8, a, p. 80. 

 The two lateral pairs are united with the stomodaeum, while the dorsal and ven- 

 tral axial pairs, representing the directives, are free, and of the two directive 

 pairs the ventral are slightly larger than the dorsal. In larvae a day or so 

 older the ventral directives have united with the stomodaeum, while the dorsal 

 are still free (plate 8, fig. 51) ; also at the stage with three complete mesenteries 

 two other bilateral pairs of mesenteries, the fifth and sixth in the sequence, have 

 made their appearance, arranged as on plate 8, fig. 52. Afterwards the dor- 

 sal directive mesenteries unite with the stomodaeum, and the larva has reached 

 the Edwardsian stage of mesenterial development presented at the time of 

 fixation (fig. 8, a-c, p. 80). 



There appears to be no resting stage in the appearance of the mesen- 

 teries between the tetrameral and hexameral condition, such as seems to char- 

 acterize certain actinians (Lebrunia), nor in the successive union of the first 

 four pairs with the stomodaeum. As already shown, however, the fifth and 

 sixth pairs remain as microcnemes for a prolonged period. 



As represented on plate 8, figs. 51 and 52, the mesenterial mesoglcea is 

 extremely thin. The cut ends of very delicate muscular fibrils can be recog- 

 nized in transverse sections, and, according to their disposition on one side or 

 other of the mesoglcea, assist in the determination of the paired arrangement. 



The further development of the mesenteries, after fixation has taken 

 place, can be easily followed through the transparent tissues of the living 

 polyp, and has been already described. 



