STUDIES IN THE EMBEYOLOGY OF ECHINODEEMS. 439 
tion of the terminals in Bipinnaria (figs. 13 and 14), and that of 
the dorso-central in Ophiurid Plutei (figs. 4 and 5), we 
can, I think, have little hesitation in identifying the cavity on 
the right of the mesentery in fig. 17 as the right posterior 
enterocoel. In any case it is quite certain that the basals are 
not related to the left enterocoel, as the terminals are ; and I 
fully agree with Carpenter (8, p. 386) and Sladen (29, p. 37), 
that this affords the strongest possible argument against the 
homology of madreporic plate of Ophiurids with that of 
Asterids, which Ludwig has attempted to establish. 
Crinoids. — Here again, as originally pointed out by Gotte 
(11, p. 395), whose account I can fully confirm, we find a 
bilateral arrangement of the primary skeletal plates, corre- 
sponding to the bilateral division of the enterocoel ; that is to 
say, five orals are developed round the left enterocoel, and five 
basals round the right enterocoel. The water-pore is at first 
unconnected with any plate, but after a while becomes sur- 
rounded by one of the orals. In his paper on Comatula, 
Barrois (5, p. 634) gives a very different account ; he begins 
with the statement that it is a recognised fact that in Eehiuids, 
Ophiurids, and Asterids the primary plates are developed 
asymmetrically, only on one side of the body. This is difficult to 
reconcile with the writings of Carpenter and Sladen above 
referred to, and is absolutely opposed to the facts related in 
this paper. Then, after pointing out that the plates of 
Antedon do not at first reach the ventral side, Barrois attempts 
to prove that at the time of the formation of these plates the 
right body-cavity is wholly dorsal, and the left wholly ventral, 
so that the plates belong solely to the right side. I consider 
this supposition to be entirely negatived by the observations of 
Gotte and myself (11 and 7). 
Eehiuids. — Although certain plates have long been known 
to exist in Echinid Plutei, no one, so far as I know, has 
traced them into connection with the plates of the young 
pentamerous Echinus. On the right side of an advanced 
Pluteus of Echinus microtuberculatus we find the 
plates shown in fig. 10 ; two of them bear pedicellarke 
