80 HENRY BURY. 
of the right side of the larva; but I may add here that in 
figs. 11 and 12 of that paper the water-pore was drawn much 
too large (a superficial hollow in the plate being mistaken for 
it), and that the presence of two pedicellariz on the second 
basal plate is much more common in the Pluteus than I then 
supposed. Outside the ring of basals, and alternating with 
them, we see the five oculars; each of them bears a pair 
of quadrilateral spines, which are not found on any 
other plates except the basals. When we compare 
these plates with those just described in fig. 34, and take into 
consideration the rarity of these quadrilateral spines, the 
various facts known about the disappearance of the primary 
tentacles, and the termination of the radial water-vessel of the 
adult in the eye-spot, on the oral side of the ocular plate, we 
can scarcely, I think, doubt that these oculars are identical 
with the five plates which lie above the primary tentacles in 
fig. 34. But these plates are developed on the left side, as are 
the terminals of Asterids and Ophiurids, which also in the 
adult embrace the terminations of the radial water-vessels; so 
that we have, as it seems to me, new and important grounds 
for accepting the homology, often suggested but never proved, 
of the oculars of Echinids with the terminals of Ophiurids and 
Asterids. 
Another consideration follows from this identification of the 
plates in figs. 34 and 36. In the younger stage the line of original 
division between the right and left body-cavities (the mesentery 
having probably disappeared already) lies somewhere between 
the basal and ocular plates—probably in the position of the 
dotted line in fig. 35, and nearly at the level of the water-pore. 
In adults these two rows of plates fit into one another, but the 
genital rachis encircles the intestine at the level of the basals 
(through which the genital ducts pass) and of the water-pore ; 
so that we have here somewhat better grounds than in Asterids 
for believing that the genital rachis marks the line of division 
between the right and left body-cavities. 
The arrangement of the water-tube and the neighbouring 
organs is almost exactly the same in Echinus microtuber- 
