440 
H. BUEY. 
(the dorsal plate may have two) as well as one or two 
spines, the number of which varies slightly. Just anterior 
to the ventral pedicellaria is a third plate bearing a spine 
but no pedicellaria. I have ascertained, by means of sec- 
tions, that these three plates lie immediately over the right 
enteroccel, and the relation to this cavity of the two which bear 
pedicellarise is shown in fig. 20. 
In one form of larva, of which I obtained but few specimens, 
there was also a terminal pedicellaria at the posterior end, and 
in Echinus microtuber culatus a small calcareous nodule, 
the remnant of the posterior end of one of the larval skeletal 
rods, occupies a similar position, and appears to develop later 
into a plate. On the dorsal side of a fairly young Pluteus 
lies a tri-radiate skeletal rod, which can be seen in fig. 8, 
where it is the only part of the larval skeleton represented. 
Later on a reticulated plate is formed round the median pos- 
terior arm of this rod, and before long envelops the water-pore, 
and renders the observation of the subjacent parts extremely 
difficult ; it also possesses a spine which is seen in figs. 7 and 
10. This plate is clearly the madreporite, and here, as in 
Asterids, we are called upon to decide whether this plate 
belongs to the right or left enteroccel, since it lies in a median 
position over what is, apparently, a schizocoel cavity. This 
question seems to me settled, by the fact that the five plates 
enumerated become the five basals (genitals) of the adult ; and 
since three of these plates are unquestionably formed round 
the right enteroccel, it seems reasonable to suppose that the 
other two are related to the same cavity. 
A comparison of figs. 10, 11, and 12 will clearly show that 
the five plates of the Pluteus become the basals of the adult. 
In fig. 11, which represents a young Echinus a few hours 
after its metamorphosis from the Pluteus, the plates in ques- 
tion have changed their positions, but are otherwise not 
materially altered; but in fig. 12 a much later stage is shown 
in which the pentamerous arrangement is more clearly 
marked ; the number of spines and pedicellarise, however, still 
remains unchanged, except that one plate has two of the 
