423 
H. BURY. 
homologous with the five primary tentacles of other Echino- 
derms. His evidence seems to me insufficient ; on p. 197 he 
refers to pi. viii, fig. 3, as proving the interradial position of 
the longitudinal vessels; yet in this figure the water-tube 
(stone-canal) is distinctly adradial, i. e. between a tentacle 
and a longitudinal vessel, and no figure whatever is given of a 
stage (supposed to precede this) in which the water-tube is 
midway between two tentacles. For my own part I have 
never seen such a stage as this, but have always found the 
water-tube to be adradial from the first, though I do not agree 
with Semon as to which adradius it occupies. 
In fig. 27 I have given the result of my observations in a 
diagrammatic form comparable to Semon’s fig. 3 (pi. viii) : 
it will be seen that in my figure we have to cross two primary 
tentacles and one longitudinal vessel in passing from the water- 
tube to the Polian vesicle ; now, according to one of Semon’s 
figures (fig. 3), we have to pass two primary tentacles and two 
longitudinal vessels ; but another of his figures (pi. viii, fig. 
2), when carefully examined, gives the same results as mine; 
and the same position is assigned to the water-tube in Baur's 
figures (6). Whatever doubts may exist on this point after 
the examination of Auricularia are easily set at rest by 
sections through young Synaptee, for in these it is no difficult 
matter to ascertain the positions of the water-tube and Polian 
vesicle, while the primary tentacles and longitudinal vessels 
are exceedingly conspicuous. It is surprising that Semon did 
not adopt this method of inquiry, which would also have set at 
rest his doubts (28, p. 305) as to which end of the hydrocoel 
formed the Polian vesicle. 
Summary and Conclusions. 
We will now summarize the facts above related, and see 
what conclusions can be drawn from them as to the nature 
and origin of the liydroccel ; in doing so we shall repeat for 
the sake of clearness some of the arguments used on p. 420. 
(1) Origin. — The hydrocoel always arises on the left side 
as a derivative of one or other division of the coelom. 
