4 SIDNEY F. HARMKR. 



which compose the ciliated ring (cr.) are to be seen at opposite 

 ends of the section. The front part of the oral surface (within the 

 area of the ciliated ring) is in the form of a depression, the 

 " pyriform organ " of Barrois. This structure is developed as a 

 cup-like involution of the epiblast, and there is no reason for 

 believing that in Alcyonidium it makes its appearance in the 

 interior of the embryo and subsequently fuses with the skin, as is 

 stated by Barrois to be the case in Lepralia (2, p. 24). Not far 

 behind the middle of the ventral surface is the aperture of the 

 large sucker (s.) or " internal sac " of Barrois ; this structure, like 

 the pyriform organ, is developed as an invagination of epiblast, its 

 nperture being much wider in earlier stages than in the embryo 

 figured. 



The alimentary canal consists of stomach {st.) and oesophagus 

 (cps.). The stomach is lined by an extremely indefinite epithelium 

 and has probably been developed by the hollowing out of the solid 

 hypoblast-mass of earlier stages. The oesophagus (perhaps deve- 

 loped as a stomodaeum) has a very fine lumen which can be traced 

 as far as the stomach. The mouth (m.) is large and is far more 

 conspicuous at this than at any of the later stages. 



There is some slight reason for believing that the region 

 immediately behind the aperture of the sucker (internal sac) 

 represents, potentially, the anal region {vide fig. 1)'. If this is 

 really the case, it is obvious that the embryo is entoproctous, and 

 that the part of the body between the posterior end of the sucker 

 and the ciliated ring represents the anal cone. 



Figs. 2 and 3 are drawn from median longitudinal sections, the 

 latter illustrating the structure of an embryo almost ready to be 

 hatched, whilst the former explains the condition of the various 

 organs in the period intermediate between the stages represented 

 in fig. 1 and fig. 3 respectively. In fig. 2 the alimentary canal is 

 seen to have acquired its maximum development. The lumen of 

 the stomach (st.) is very conspicuous, although the epithelium 

 which bounds it is by no means distinct, at this or at any other 

 stage. This epithelium may consist of a mass of yolk-spherules 



' The distinctness of the anal region has unfortunately been exaggerated in the 

 fitjuie. 



