108 HENRY BURY. 
originally posterior, is now the most anterior of all. It would 
be difficult and tedious to describe in full the changes of 
position which the other pouches undergo, but in fig. 40 [ 
have placed numbers against them so that a comparison with 
5, figs. 2—6, will show with which arm-rudiment each unites. 
Much of this complicated process no doubt is due to ontogenetic 
causes, and is unconnected with phylogeny, but the movement 
by which the primarily anterior tentacle (see 5, figs. 2 and 4) 
passes all round the cesophagus and finally is embraced by the 
second terminal plate is so peculiar, and so exactly corresponds 
in direction with the movement of the hydroceel in Asterina, 
that it can hardly be without significance. In both larve it 
will be noticed that the pouch which lies immediately anterior 
to the water-tube subsequently unites with that terminal plate 
which is the most posterior of the longitudinal series in the larve 
(5, “ Terminal 5 ” in fig. 2). 
The two movements spoken of above are combined in such a 
manner in Ophiurid Plutei that at the end of metamorphosis 
the original antero-posterior axis, still traceable with the help 
of the arms of the Pluteus, passes distinctly to the left of the 
water-pore (compare 9, figs. 5 and 6); this is due, as already 
explained, to the first movement of the above description, while 
the second movement is responsible for the fact that the former 
left side of the stomach is at this stage (if we may trust the 
external evidence of the calcareous plates) entirely on the 
original ventral surface. In Crinoids, on the other hand, it 
would seem that the first movement has little effect, while the 
second carries even the pore far over to the left of the original 
‘‘ Medianebene.” An intermediate condition between these 
two is seen, as already indicated, in Bipinnaria (fig. 22). 
As previously mentioned, the position ascribed to the dorsal sac 
and organ in the bilateral ancestor will be discussed especially in 
connection with the relation to the Enteropneusta; but it is worth 
while here to show that, if that position be admitted, the move- 
ments we have just been considering will bring these organs 
exactly into the relation to surrounding structures which is 
observable in adult Echinoderms. The dorsal sac has so far 
