78 HENRY BURY. 
as well as, in aless degree, in Ophiurid Plutei and Brachio- 
laria; but the fact that one or two of my KEchinid Plutei 
hardly showed it at all, and that Seeliger was not troubled with 
it at all in Antedon, indicates that, as that author suggests 
(30), it is probably pathological. In all larve kept under 
satisfactory conditions it soon clears off, and the tissues return 
to their normal condition. 
Although this histolysis has prevented me from following 
the details of the anatomy of the youngest Echinid to my 
satisfaction, yet we may state positively that the essential 
relations of the organs are not much altered during metamor- 
phosis. If any mesentery exists at this stage (which is doubt- 
ful) it must still occupy the same position as before—parallel 
to the ambulacral surface, at the level of the water-pore ; 
but since the total bulk of hydroccel and left body-cavity 
combined is far greater than that of the right body-cavity 
alone, it follows that the divisional line between the two cavi- 
ties lies far on the aboral side of the equator of the young 
Kchinid. Its approximate position is shown by a dotted line 
in the diagram, fig. 835, which will be presently described in 
connection with the skeleton. 
In a previous paper (5) I described the position of the 
basal plates in the Pluteus; though they do not form such a 
regular longitudinal series as in Asterids, yet all (except 
perhaps the madreporic plate) are formed over the right 
body-cavity. Most of them possess spines, which when first 
formed usually seem to end in three points, but when fully 
formed are seen to be quadrangular and to terminate in four 
points, not always of equal length; two of them are shown in 
fig. 32. 
Besides these plates a number of others are formed on the 
left side, round the base of the “amnion,” into which their 
spines project. They are most difficult to study in the Pluteus, 
and I have not yet determined, in spite of much time spent 
on them, their order of development; apparently a large 
number of them are formed within a few hours of one another. 
In a young post-larval stage their positions are more easily 
