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THE METAMORPHOSIS OF ECHINODERMS. 67 
anterior end of the dorsal mesentery is left rather than median, 
though not so much so as the water-pore. In this way a 
portion of the right posterior body-cavity comes to lie close 
behind the dorsal sac, with which, owing to the thinness of the 
intervening wall, it is rather liable to be confused in sections. 
The two, however, are absolutely distinct. 
The hydroceel has thicker walls than before, especially in the 
five primary pouches, which now appear ; and the rudiments of 
the water-tube and dorsal organ are also distinguishable, 
though I do not propose to deal with the details of their 
development in the present paper. 
At this stage also the first rudiments of the skeleton appear 
—namely, five terminal plates, lying parallel to the mesentery, 
on its left side ; two are dorsal, one is at the extreme posterior 
end, and two are ventral; the latter are shown in fig. 18. 
There are also a few plates, not shown in this figure, lying over 
the hydroccel. The madreporite appears very soon after the 
terminals; but except for the fact that all the terminals lie 
over the left body-cavity, the skeleton has no special interest 
in our present inquiry. 
The completion of the water-tube follows closely on the 
stage just described. It runs from the hydrocel up to the 
immediate neighbourhood of the water-pore (where it remains 
permanently open to the anterior enteroccel) almost exactly 
along the line occupied at any earlier stage by the mesentery 
separating the anterior and posterior body-cavities. It is accom- 
panied throughout its course by the dorsal organ, which now 
almost encircles the hydroccel, and ends anteriorly under- 
neath the dorsal sac, as we shall presently see is the case in 
other Asterid larve as well (see fig. 19). Its tubular nature is 
well marked at this point, as well as round the hydroceel (fig. 20), 
but where it accompanies the water-tube it is usually very 
narrow, and its lumen, if present, is very hard to detect. With 
its further development we have no present concern. 
Almost simultaneously with the completion of the water- 
tube, the separation of the hydroccel from the anterior body- 
cavity is effected. Fig. 21 is a lateral view of a larva at this 
