THE METAMORPHOSIS OF ECHINODERMS. 81 
culatus as in Dorocidaris (27), except that in the oldest speci- 
men I have cut (diameter 3°5 mm.) the ‘‘ espace sous-madré- 
porique” of Prouho does not extend down the side of the 
water-tube. 
In spite of the unsatisfactory histological condition of my 
youngest radial specimens, and the gap which still exists 
between them and the next stage obtained, the relations of 
the parts in the Pluteus to those in the adult can be fairly 
easily followed. 
The pore or pores of the madreporic plate and the upper 
end of the water-tube open in all stages into an ampulla 
(figs. 830 and 39), which is continued down into a canal lying 
alongside the water-tube, and enclosing the axial organ. 
This is the ‘‘canal aquiferé annexe” of Prouho, and being 
derived from the anterior enteroceel is apparently homologous 
with the axial sinus of Asterids. (Fig. 39, “axial canal.’’) 
Lying under the madreporic plate, on the aboral side of 
the water-pore, is a closed vesicle into the floor of which pro- 
jects a portion of the dorsal organ, which passes round the 
water-tube into the axial sinus at a lower level than the section 
drawn in fig. 39. This closed vesicle is Pronho’s “ espace 
sous-madréporique,” and the portion of the dorsal organ pro- 
jecting into it is his “ processus glandularis.” Its position is 
exactly that of the “pulsating vesicle” of the Pluteus (for 
what is on the right of the pore in the larva is on its aboral 
side in the adult), and its obvious similarity to the dorsal sac 
of Asterids justifies the conclusion already arrived at that 
this sac is homologous with the “ pulsating vesicle” of the 
Pluteus. 
Fig. 39 very well illustrates the general relation of these 
parts to one’another and to the intestine and ovary, though 
the junction of ampulla and axial canal takes place at a higher 
level, and the connection of the latter with the dorsal organ 
(as already mentioned) at a lower level. The water-pores are 
clearly adradial, being on the left of the ovary, which is inter- 
radial. The water-tube in this figure appears to be on the 
left side of this interradius, but at its oral end it opens into 
VOL. 38, PART 1.—NEW SER. F 
