124 HENRY BURY. 
suggest that the mouth, after first moving into the left side, 
has undergone a secondary change of position, accompanied 
of course by the water-vascular ring. 
To some extent this is parallelled by the Spatangidea, and 
(possibly) by Actinometra; but the case of the Holothurians 
presents certain marked pecularities. In the first place the 
secondary migration of the mouth has apparently been such as 
exactly to retrace the line of the original movement—that is to 
say, it has occurred in the direction of interradius A (compare 
figs.47 and 48); though in this it does not differ very widely from 
the Spatangidea, in which the secondary movement has been 
towards radius I (see 14, pl. xiii, fig.8). Secondly,it has not been 
a simple movement; for that, while it would reduce the length of 
the mesentery of the water-tube, would still leave the same 
angle between this and the mesentery of the stomach and intes- 
tine which we have found in other Echinoderms. But in 
Holothurians these two mesenteries are nearly in the same 
straight line; and this can, I think, only be accounted for 
by supposing that the torsion of the water-vascular ring 
which we traced in the common ancestor has been reversed 
and undone during the secondary movement, so that this 
ring has returned very nearly to the position assumed for 
it in the bilateral ancestor (fig. 45), in which the dorsal 
mesentery may be said to lie in interradius A (the interradius 
of the water-tube). One great difference, however, exists 
between this secondary Holothurian ancestor and the ancestor 
shown in fig. 45. In the latter the body-cavities do not run 
forward on to the csophagus, but in the former, during the 
time when, in common with other Echinoderms, its mouth was 
on the left side, the left body-cavity learnt, so to speak, to 
surround the «esophagus and form a mesentery for the 
water-tube; and this peculiarity, once acquired, was not lost 
during the secondary changes, but is still traceable in the larva 
of Synapta; and it accords very well with this view that, 
whereas in other forms (compare fig. 17) a dorsal as well as 
a ventral horn of the left body-cavity is observable, in Synapta 
the ventral horn alone is conspicuous; this, however, is a point 
