58 HENRY BURY. 
Echinoderms. The animal is looked at from the aboral pole, 
with the interradius of the water- pore directed away from the 
observer, this interradius being regarded as anterior. The 
ray immediately to the left of thisis designated No. I, the next 
beyond it No. II, and so on, that immediately to the right of 
the anterior interradius being No. V. The interradii may be 
conveniently marked by the letters A, B, C, D, E—A being 
the anterior interradius, and the order of succession being the 
same as in the case of the rays. This system of numbering 
and lettering is adopted in fig. 14, in which the arrangement 
of the hydroccel in the “pupa” is diagrammatically shown. As 
there seen, the water-tube is adradial, being nearer to radius 
V than to radius I, from which it is separated by one of the 
tentacles. The polian vesicle is also adradial, lying in inter- 
radius B, close to radius II. 
Semon’s figure (82, pl. ii, fig. 2) of these parts is extra- 
ordinarily inaccurate; not only, as I pointed out before (5, 
p. 22), is it out of harmony with his statement of the position 
of the water-tube in Auricularia, but this tube is represented 
on the wrong side of the polian vesicle ; seen from the side, as 
he has drawn it, it should be on the right of this vesicle (com- 
pare fig. 14), but he has represented it on the left ! 
In none of my specimens can I detect that curious relation 
of the tentacles to the radial vessels which Ludwig describes 
in Cucumaria (16, p. 183) ; but if, as is probable, it obtains in 
Synapta also, in later stages than I have examined, it may 
help to explain one curious discrepancy between Synapta 
(and probably Holothurians as a whole) and other Echino- 
derms. In all Echinoderms, at any rate in young stages, 
the water-tube is adradial, but in most forms it is nearest 
to the left side of the interradius in which it lies; in the 
larva of Synapta, however, and of Cucumaria (16, pp. 187, 188 ; 
17, p. 611) it lies, as shown in fig. 14, on the right of this 
interradius. Now if the tentacle which separates it from 
radius I belongs to this radius (as Ludwig says), it may be 
that the precocious development of this tentacle has pushed 
the water-tube (in appearance at least) somewhat out of its 
