120 SIDERASTREA RADIANS. 



In the serial transverse sections from which figs. 51 and 52, on plate 8, 

 were taken, the stomodseal ectoderm appears to be continued uninterruptedly 

 as the mesenterial filaments down the edge of the two lateral pairs of com- 

 plete mesenteries on their becoming free; but the ventral and dorsal direct- 

 ives (ill, iv), which are only attached to the stomodseal wall for part of their 

 length, are without such modified tissue along their free edge. 



The continuity of the stomodseal ectoderm with the mesenterial fila- 

 ments in the various larvae of Siderastrea, and the close histological simi- 

 larity of the two, seem at first sight undoubted evidence of the ectodermal 

 origin of the latter, especially when it is found that filaments are not present 

 on mesenteries which do not reach the stomodseum, or do not extend as far 

 as its inner termination. In the larvse of many other Actiniaria and Madre- 

 poraria, however, it has been found that filaments may appear on mesen- 

 teries before they reach the stomodseum, and even in complete mesenteries 

 an interval of undifferentiated endoderm often occurs between the ter- 

 mination of the stomodseal ectoderm and the filaments. From observa- 

 tions on other corals I consider that the mesenterial filaments arise from the 

 larval endoderm independently of the stomodseal ectoderm, but that continuity 

 is early established in the case of those mesenteries which unite with the 

 stomodaeum (1902, p. 476). 



The internal cavity of the larvse is very limited in extent, the endoderm 

 nearly filling the whole chamber. In the youngest examples mere slits 

 represent the lines along which the polypal cavity will be formed later. The 

 cells of the endoderm are much vacuolated, and contain numerous Zooxan- 

 thellss scattered throughout. The lining of the mesenteries and also of the 

 intervening portion of the column wall is not arranged as a simple epithelial 

 layer such as characterizes the adult. Within most of the intermesenterial 

 spaces the endoderm is greatly thickened, and in transverse sections stands 

 out as a very distinct triangular projection into the ccelomic cavity, leaving 

 only a narrow slit between itself and the mesenterial lining. The vertical 

 projections are the " Vorsepten " or prosepta of von Koch (1897) ; they are 

 the persistent parts of the mass of endoderm which at an earlier stage occu- 

 pies the whole interior of the early larva. On plate 8, fig. 52, taken from a 

 rather late larva, the prosepta are still conspicuous, and some are associated 

 with rudiments of the mesenteries ; the mesenterial endoderm is still greatly 

 thickened, and the central cavity is beginning to enlarge. 



In an earlier paper on the larva of the actinian Lebrunia coralligens 

 (1899), I nave shown that most anthozoan larvse are for some time nearly 

 solid, owing to the enormous development of the endoderm. There are, how- 



