444 
H. BURY. 
rior and posterior radii by means of the tentacular pouches, 
before they have selected, so to speak, their respective ter- 
minals, would only involve us in confusion. 
In a previous paper (7, p. 294) I imagined that the arrange- 
ment found in Antedon, in which the mouth and anus are 
actually in the same interradius in the adult, was arrived at by 
a secondary shifting of the anus. This appears to be Ludwig’s 
idea (14, p. 54), and is also advocated by Barrois for Antedon 
(5, p. 638). Now, however, I am compelled to regard this 
position of the anus as primitive, though of course it is still 
possible that in this particular group it may have been 
secondarily derived from such a condition as is found in 
Asterids. 
Of the view here advanced, that the mouth, anus, and water- 
pore belong primarily to the same interradius, some further 
support is afforded by an examination of the Echinids. Turn- 
ing to fig. 10, already described, we can without difficulty trace 
a series of five basal plates, beginning with the madreporite as 
the most anterior, working backwards by way of the dorsal 
pedicellaria, and ending up with the plate which bears a spine 
but no pedicellaria. These plates are of course interradial, 
but by taking tbe interspaces to represent the radii, we shall see 
that there is some reason for thinking that here again the 
mouth, anus, and water-pore occupy the same interradius. 
Another point to be noticed is, that within this interradius the 
water-pore and the mouth frequently occupy adradial positions. 
In Ophiurids they clearly lie on opposite sides of the same 
interradial plate (figs. 2 and 3). The same is true of Echinids, 
though it is not evident from my figures ; for in them too the 
madreporic plate is at first situated in front and to the right of 
the pore. In Crinoids, again, this fact is very distinct (7, 
fig. 45), and in these, as in Ophiurids, the adradial position of 
the water-pore is long marked by its excentric position in the 
madreporic plate. 
Among Asterids I have not been able to obtain any clear 
evidence of this, for the madreporic plate generally seems to 
arise opposite the pore, and just to the right of it (fig. 14), or 
