THE METAMORPHOSIS OF ECHINODERMS. 107 
details are a good deal masked by what appear to be secondary 
and purely ontogenetic processes. 
Hitherto, for simplicity of description, I have regarded the 
stomach as fixed and the cesophagus as undergoing movement ; 
but seeing that the mouth is always on the ground, to which 
the animal adheres by its tentacles, it might have been more 
accurate to regard this as the fixed point, and speak of a move- 
ment of the stomach across the base of the cesophagus. One 
of the first results of this movement is to throw the water-pore 
and front end of the mesentery across to the right side, as seen 
in fig. 46, and this finds its counterpart in ontogeny in the 
position of the water-pore in a late Ophiurid Pluteus (fig. 43). 
But a close comparison of these two figures will show us that 
in the Pluteus some other changes have occurred as well. The 
mouth was from the first ventral, and in order that the left 
body-cavity may surround it, there must be a movement of 
this cavity towards the ventral side. There are in fact two 
movements, one tending to make the left side of the stomach 
anterior, and the right side posterior (as in fig. 46); and the 
other pushing the left side on to the ventral, and the right 
side on to the dorsal surface. The former would lead to great 
asymmetry, which the latter would to some extent counteract ; 
and it is just possible that we have here the reason of this 
second movement, though as we are as far as ever from the 
reason of the first, this suggestion is not of much value. 
Now if we consider carefully what the nature of this second 
movement is, we shall see that it involves exactly that ‘ un- 
screwing” of the stomach from the cesophagus which we have 
already seen in Brachiolaria, and which we now see must also 
take place in Ophiurid Plutei. An examination of the hydrocel 
pouches in this larva points in the same direction. At first they 
are arranged as a longitudinal series along the left side of the 
cesophagus (fig. 40); then one by one they pass across the 
dorsal surface of this organ, and encircle it; and finally, at the 
time of metamorphosis the water-pore also moves forwards and 
to the right (by a process already explained), and we reach the 
rather curious result that that tentacular pouch, which was 
