6 E. W. MacBRIDE. 
When we review the accoant that is here given of these post-larval Holothurians, 
we are, I think, confronted with a certain number of Echinoid features. 
The almost complete dorsal cuirass of overlapping plates certainly suggests the 
corona of an Echinoid, especially if we recollect that, in the earliest Echinoids, the 
number of rows of plates in the corona was indefinite in number and the plates over- 
lapped. 
The closed axial sinus is another Echinoid feature. 
Some investigators have made an attempt to show that Holothurioidea are 
widely separated from the rest of the Echinoderma. From the development of Synapta 
digitata, the only Holothurian in which the life history has been thoroughly worked 
out, it has been argued that since in this form the primary evaginations of the water- 
vascular rudiment give rise to the buccal tentacles, whilst the radial canals owe their 
origin to secondary evaginations alternating with these, the radial canals of 
Holothurioidea are not homologous with those of other Echinoderms. But in our 
specimens the canals occupying the interior of the tentacles can be traced into contin- 
uity with the radial canals. The fact is that Synapta is about the worst form that 
could have been chosen to represent the Holothurioidea. Its radial water-vascular 
canals are only trausitory larval structures, and its buccal tentacles not only spring 
directly from the water-vascular ring, but, in contravention of the rule which prevails in 
all the other groups of Holothurioidea, their number is no longer a multiple of five. 
The functional importance of the buccal tentacles leads to their early appearance in 
ontogeny, before the transitory radial canals appear. Professor Ludwig, who worked out 
the early development of Cucumaria planci (4), points out that in this form the first 
tentacles spring from the radial canals. 
If we compare the youngest post-larval form with the Auricularia which was 
described in the beginning of this paper, we see that the alimentary canal in both 
is very similar. The long stomach obviously corresponds in each case, as does the 
short intestine. The short thick cesophagus of the post-larval form corresponds to the 
inner part of the cesophagus of the Auricularia, whilst the shallow outer part becomes 
(as we know from the life-history of other Auricularia) the oral vestibule. 
The chief difficulty in the way of deriving Holothurioidea from a primitive form 
of Echinoidea has lain in the difference between the Lehinopluteus and Auriculuria 
larvee, and the apparent retention by the latter of more primitive features than 
those shown by the Lehinopluteus. I have shown elsewhere (5) that the stomach 
and intestine of the Kehinopluteus are directly converted into the stomach and 
intestine of the young Echinoid. Further, in our specimens the stomach remains 
unchanged in shape as growth proceeds, and the looping of the alimentary canal 
is due solely to the growth in length of the intestine. This is also true of 
the Hehinopluteus. The “amniotic space” in the Lehinopluteus inside which the 
oral dise of the young EKchinoid is formed corresponds to the oral vestibule of 
the Auricularia—the difference between the two cases being that in the latter 
