THE METAMORPHOSIS OF ECHINODERMS. 75 
c. ECHINIDs. 
My studies of the internal anatomy of Echinids (both larve 
and adults) have been chiefly made on Echinus micro- 
tuberculatus; but so far as other forms have been examined 
all Regular Urchins follow much the same plan of develop- 
ment. 
It is unnecessary to repeat here the description which I 
gave in a previous paper (9) of the origin and general arrange- 
ment of the parts of the cclom in this group; but attention 
may be called to the migration of the hydroccel along the 
left side of the stomach. Originating at the junction of ceso- 
phagus and stomach, it pushes its way backwards over the 
surface of the latter till it comes to lie about in the middle of 
the left side, where it forms a ring through which (at a later 
stage) the cesophagus of the adult grows—the larval cesophagus 
not being permanent. Round this hydrocel ring the left 
posterior body-cavity grows in the form of a crescent, the 
dorsal and ventral horns of which eventually unite and fuse 
together anteriorly. I believe that this fusion occurs along 
the line of the water-tube, which, as in Bipinnaria, lies close 
against the wall of the stomach ; but I have not yet been able 
to obtain decisive proof of this. 
The limits of the body-cavities in older larve are extremely 
difficult to determine, but they appear to be separated by a 
mesentery starting just behind the water-pore, and running 
back along the middle dorsal line to the extreme posterior 
end, where it turns and runs forward along the ventral surface 
of the intestine. I have not been able to trace any communi- 
cation between the cavities between the anus and stomach, 
such as occurs in Asterids, though there are some indications 
that it occurs just before metamorphosis. 
The general arrangement of the left anterior enteroccel and 
the organs adjacent to it was described and figured by me in 
a previous paper (9, p. 18, fig. 9); but some further details 
may be added here. 
The “ pulsating vesicle ” arises at a fairly early period from 
