294 MK. J. E. DTJEEDEN ON 



When the sections are perfectly horizontal the reflection at 

 first is equally developed all round, except at the two points 

 mentioned, and is continued above in four distinct bands (fig. 17). 

 The extent of the reflection in the non-extruded larvae (fig. 211 

 should be compared with the condition before and after disinte- 

 gration in the others (figs. 11, 22). 



The sections oi Lelrunia prove that such a condition as that 

 figured by McMurrich — the stomodseum open at one end and 

 the lips partly turned back, and with a free portion of the 

 lining reflected outside the opposite end — is probably due to an 

 obliquity in the sections to the vertical axis of the larva, but 

 it would, of course, also be brought about if the stomodaeum 

 terminated at one end earlier than at the other. 



The reflection has the significance of constituting the point of 

 continuity between the oesophageal lining and the mesenterial 

 filaments, and occurs also in the adult condition of most 

 Zoantharia. 



At the present time one of the most important problems in 

 Actinozoan morphology concerns the origin, from one or other of 

 the two embryonic layers, of the mesenterial filaments. The 

 facts observed in every case are as follows : — Histologically the 

 lining of the oesophagus very closely, though perhaps never com- 

 pletely, resembles the mesenterial filaments ; and in the perfect 

 mesenteries of all adult polyps the two are in absolute continuity. 

 In so far as the filaments resemble the stomodseal liaing do they 

 differ from the ordinary epithelium of the gastro-coelomic cavity j 

 everywhere they off'er a very strong histological contrast to the 

 latter. 



Embryological evidence is here of the greatest value. Erom 

 this standpoint the question has been approached by Prof. E. B. 

 Wilson (1884") for the Alcyonaria, by Prof. H. V. Wilson for 

 the Madreporaria (1888), and by Prof. McMurrich (1891) and 

 others for the Actiuiaria. In his paper on " The Mesenterial 

 Eilaments of the Alcyonaria," the first mentioned investigator 

 arrives at the conclusion that the filaments of the two dorsal 

 mesenteries in that group are ectodermal derivatives, that is, 

 downgrowths of the lining of the stomodseum, and are homologous 

 with the Elimmerstreifen or ciliated streaks of the Actiniariaj 

 while the six ventral filaments are endodermal in origin, and 

 correspond with the Driisenstreifen of the Actiniarian filament. 



