THE METAMORPHOSIS OF ECHINODERMS, 57 
hydroceel ring, facing it, is directed decidedly towards 
the left side. 
This arrangement is of brief duration and easily overlooked, 
but, as will be shown in the second part of this paper, it affords 
an important clue to the probable phylogeny of the Holo- 
thurians. 
In stage A the radial vessels and tentacles lie nearly in the 
same plane; but in stage B the latter turn up alongside the 
outer wall of the atrium, while the former bend back parallel 
to the long axis of the stomach, and grow rapidly towards the 
posterior pole of the larva. On the outer side of each runs a 
two-layered prolongation of the wall of the atrium; but this, 
and the ectodermic covering to the tentacles, are so well 
known from previous descriptions that they need not be dwelt 
on here. 
Before describing the positions occupied by the parts of the 
hydrocel in the fully-formed pupa, it will be well to come to 
some conclusion as to the nomenclature of the rays. 
In adult Holothurians it is usual to speak of the rays as 
right and left dorsal, right and left ventral, and median 
ventral; the water-tube being in the dorsal interradius. 
Ludwig applies these terms to the young Cucumaria; but in 
the young Synapta their employment might lead to some con- 
fusion. Up to the end of stage B, and sometimes even in the 
pupa, the original right and left sides of the larva are distin- 
guishable by means of the groups of calcareous discs at the 
posterior end, and this enables us to see that the water-pore 
remains, as in Auricularia, decidedly to the left of the middle 
line ; and though its exact position in older stages (fig. 9 and 
later) is rather hard to determine (and perhaps variable), it is 
never truly median. 
Again, the radial vessel which is nearest to the ventral 
median line in stage B (fig. 5) is certainly not the median 
vessel of the adult, but (as I believe) the left ventral. Since, 
then, the two planes of symmetry (larval and adult) do not 
correspond, it may be better to apply that nomenclature to the 
rays which has been adopted on morphological grounds in other 
