118 HENRY BURY. 
Accepting for the present the supposition of a preoral 
sucker in the bilateral ancestor, it would seem that it ought, 
in the pentamerous form, to lie in the same interradius as 
other organs (water-pore, water-tube, dorsal organ, &c.) 
belonging to the same region; and here, I think, we meet 
with the one serious objection to this view, which involves 
the homology of the stalk by which Asterina is fixed with that 
of Antedon. The latter, as we have seen, is either in radius 
V, or, as I think, in interradius A, while the former, ex- 
ternally at least, is in the next interradius (E). It is true that 
it has its roots, so to speak, in interradius A, since it is there 
that we find the remnant of the anterior body-cavity, which 
before metamorphosis extended far into the przoral lobe; and 
the whole stalk has undoubtedly a very oblique aspect in 
Asterias rubens, though I have not satisfactorily traced 
its internal relations. It is to be hoped that MacBride, in 
his detailed account of Asterina, will give us some explana- 
tion of this point, which at present seems to me opposed to 
the homology he supports. 
Thus the evidence of the existence of even a sucker in the 
common ancestor is extremely weak, and it is worth while 
to see whether the supposition that the Pelmatozoa became 
fixed after the change of symmetry, instead of before it, is not 
at least as probable. 
Let us imagine that a sucker arose in the interradius of 
the water-pore, somewhere between that pore and the mouth; 
how or why it arose I cannot attempt to determine (expla- 
nations of this kind are, indeed, very seldom possible), but 
at least I see no a priori objection to the suggestion. The 
selection of this particular radius may have been purely 
a matter of chance, but the eccentric position of the mouth in 
the Cystid larva of Antedon, as well as certain peculiarities 
in the ambulacral fields—‘‘ hydrospires palmées ”’ of 
Barrande (2)—of some Cystids suggest that perhaps this 
radius may have been rather different from the rest in early 
stages of pentamerism—possibly, even, it may have been (as it 
still is morphologically) the anterior interradius, and loco- 
