84 HENRY BURY. 
which the calcareous plates of the Pluteus undergo at the time 
of metamorphosis, showing that the plates developed over the 
right body-cavity (radials and dorso-central) pass up on to the 
dorsal surface of the larva, while those over the left body-cavity 
(terminals) pass on tothe ventral side. At the same time the 
madreporic plate is pushed forwards to the anterior end (finally 
resting to the right of the middle line, see fig. 43), while the 
mouth assumes a more decidedly ventral position, turning as 
it does so towards the left (fig. 43). 
All these changes in the skeletal plates are very easily 
followed on the living Pluteus ; but a question of much more 
importance to us now is, how far do the body-cavities follow 
this movement—how far does the longitudinal mesentery 
remain parallel to the lines of the radial and terminal 
plates ? 
It is on this point that my sections have most conspicuously 
failed; but fortunately enough evidence remains to place the 
matter (bearing in mind the analogy of Asterids and Echinids) 
beyond any reasonable doubt. When the radial plates of the 
dorsal side first begin to move across to the left, the edge of 
the right body-cavity is still plainly visible in external views, 
and certainly follows their movement as long as it can be traced 
—that is, nearly to the stage shown in 5, fig. 5. Similarly, on 
the ventral side the edge of the left body-cavity can be seen 
passing, still parallel to the line of the terminal plates, over to 
the right side (fig. 42). Between this and the first truly penta- 
merous form, I have no observations worth recording ; but in 
this stage (5, fig. 6) I find in sections traces of a mesentery 
running round the edge of the stomach, still parallel to the 
lines of the skeletal plates, though now at right angles to the 
axis of the cesophagus (fig. 41). I cannot trace it all round 
the body—indeed it appears to be in a very fragmentary 
condition at this stage—but enough remains to make it 
almost certain that we have here a remnant of the larval 
longitudinal mesentery, which therefore in Ophiurids as in 
other Echinoderms assumes in the pentamerous form a trans- 
verse position. 
