THB EDWAEDSJA-STA&E OF LEBEFNIA. 281 



cellular mucous glands witii nearly clear contents ; cilia are not 

 obvious in material preserved in the same manner and at the 

 same time as the larvae ; and practically no increase in thickness 

 of the layer has taken place. A weak ectodermal musculature 

 occurs on the tentacles, oral disc, and uppermost part of the 

 adult column. 



The nematocysts of the columnar outgrowths (pseudotentacles) 

 in the adult are of a dilFerent kind from those already described^ 

 being oval and much shorter, with a loose, irregular spiral 

 thread. 



2. The Archenteron and Formation of tJie (Esophagus. 



It is remarkable that in all the larvae taken from the preserved 

 adult there is no external aperture (PL 19. fig. 21). This is also 

 the case with the extruded larvae at the time they are set free 

 (fig. 15), but a mouth is established very early after as a result 

 of an infolding of the disca] ectoderm. From this it would seem 

 that the structure in Lebrunia, hitherto spoken of as the 

 stomodseum, must, prior to its communication with the exterior,. 

 be regarded as an endodermal -lined archenteron, for there are 

 important morphological differences between its earliest and 

 latest conditions. 



If the discovery of earlier stages should prove that at the 

 blastula-stage of development a primary archenteric invagination 

 is established, then its external opening — the blastopore — has 

 become closed, and only later another aperture is produced, over 

 evidently the same spot, and this persists as the mouth. The 

 entire tube thus formed, partly from the archenteron and partly 

 from the secondary invagination, persists as tlie oesophagus of 

 the adult. 



Fortunately, a fairly complete series of preparations has been 

 obtained showing the different stages in this latter transforma- 

 tion. In the non-extruded larvae (fig. 21) the blind archenteron is 

 easily distinguished by means of its deeply-staining lining ;; 

 outwardly it extends as far as the external ectoderm, its narrow 

 lumen recognizable all the way. The supporting lamella is seen 

 to be invaginated for a considerable distance, but does not extend 

 transversely. The ectodermal cells immediately over the arch- 

 enteron are rather loosely arranged, and zooxanthellse occur 

 amongst them. It appears as if all that is needed to bring about 



