88 HENRY BURY. 
(6) Perrier’s description (26) seems to leave little room for 
doubt that the genital cords start at the level of the transverse 
mesentery. They grow out, according to him (26, p. 202), 
from the oral end of the dorsal organ, and pass along to the 
arms in the septum which separates the oral and aboral arm- 
cavities—a continuation (26, pl. ix, figs. 62 and 62) of the 
transverse mesentery. 
(7) In my former figures (4) 1 represented the stalk of the 
larva as lying in radius V (compare fig. 14 of this paper). It 
is extremely difficult, in the absence of radial plates, to deter- 
mine its exact position, but I am strongly disposed now to 
believe that its true position is adradial—close to radius V, 
but actually in interradius A. There is some indication that 
it undergoes a change of position during metamorphosis; but 
further investigation of this point is required. 
Part I].—PHYLOGENY. 
Interesting as are the problems involved in the history of 
the probable bilateral ancestor of Echinoderms, they have 
very little to do with the subject of the present paper. The 
relation of this ancestor to the Enteropneusta will indeed 
be briefly discussed later on; but for the present we may 
confine our attention to the first appearance of radial sym- 
metry, and the changes represented in ontogeny by the meta- 
morphosis. 
On this point many opinions have been expressed in the last 
few years, some of them of a highly speculative character; but 
few attempts have been made to trace these changes in detail 
from an embryological standpoint. The more general sketches 
of phylogenetic possibilities, and expressions of opinion founded 
on observation of post-larval and adult examples only, can, 
where they call for any special remark, be more conveniently 
dealt with in connection with those details of my views with 
which they are immediately connected ; but those accounts 
which endeavour to give a fuller explanation of the meta- 
