THE EDWAEDSIA-STAGE OF LEBRUNIA. 283 



tion and the roof of tlie archenteron, is now absorbed or broken 

 down. The floor of the invagination is still represented laterally 

 by the very marked indentation on each side, the actual point of 

 union of the two layers being in the neighbourhood of the lower 

 edge. Neither in this section nor in any of the later stages can 

 the actual union of the invaginated ectoderm and the archenteric 

 endoderm be recognized by any histological difference. The 

 lumen throughout is now much wider than hitherto, but soon 

 narrows again. 



In all the other extruded polyps of which I possess vertical 

 sections the stage reached is later than the above; the stomo- 

 dseum has straight walls, and no indication is afforded of any 

 double origin of its lining. 



Strictly, then, only the upper, ectodermal-liued region of the 

 gastric funnel in Lelrunia is homologous with the stomodseum 

 or fore-gut of the Enterocoela ; the lower region, lined with 

 archenteric endoderm, is equivalent to a portion of the mesen- 

 teron or mid-gut of the higher Metazoa. "While such conditions 

 as those revealed in fig. 21 (PI. 19), where the archenteric endo- 

 derm is in absolute continuity with the filamental tissue, strongly 

 suggest that the mesenterial filaments along the free edge of the 

 mesenteries are but the banded continuations of the enteron. 



On account of the seeming double origin of the gullet m 

 Lehnmia, the term oesophagus is preferable to the term stomo- 

 dseum usually employed in Actinozoan literature fortius structure. 

 The former, as used in the Enterocoela, implies merely the first 

 portion of the adult gastric canal, without any reference to the 

 embryonic origin of its lining, though usually it possesses both 

 an ectodermal and an endodermal portion ; while the latter is 

 the term employed to designate the extent of the invaginated 

 ectodermal lining of the gastric canal. The justification for the 

 employment of stomodseum in the adult Actinozoon rests upon 

 the fact that, in general, the lining of the gullet is wholly 

 derived from a primary ectodermal inturning, but it is obvious 

 from the details just given, that this may not always be the 

 case. 



We may thus summarize the formation of the oral aperture 

 and oesophagus: — An invagination of the external ectoderm 

 takes place directly over the outer extremity of the archenteric 

 tube, and at the same time the layer undergoes a considerable 



