54 HENRY BURY. 
the thick-walled portion and forms the inner wall of the 
hydroceel ; while the other one (the dorsal), which is Ludwig’s 
“‘ Madreporenblase,”’ is, in my opinion, the homologue of the 
anterior body-cavity in other Echinoderms (see figs. 1 and 2; 
and 5, figs. 22—25). The two cavities are rapidly pushed 
asunder by the formation of a second thick-walled tube, which 
almost immediately becomes continuous on one side of the 
“ Madreporenblase ” with the pore-canal. This new tube I 
regard as the true water-tube, though Ludwig, under the name 
of * Steinkanal,’’ confuses it with the pore-canal (for the dis- 
tinction see 5, p. 21). 
No one from the study of Auricularia alone can say that 
one of these two cavities (“ Madreporenblase ” and hydroceel) 
is more primitive than the other—they are parts of the same 
primary coelomic pouch. Moreover, had Auricularia been 
opaque—had I been forced to rely on sections alone, it is 
very probable that I should have overlooked this division (as 
previous observers had done), so rapidly does it take place, and 
regarded the ‘“ Madreporenblase ” as a later outgrowth of the 
water-tube. With all regard to Ludwig’s admirable care in 
research, I cannot at present feel satisfied that he has not 
overlooked similar changes in Cucumaria. 
To sum up, we find in Auricularia two cavities and a tube 
connecting them having precisely the structure and relations 
of the anterior body-cavity, hydroccel, and water-tube in other 
Echinoderms ; and to my mind it is far easier to believe that 
the ‘‘ Madreporenblase ” of Auricularia is the anterior 
body-cavity, than to admit that a cavity which is present in 
all other Echinoderm larve yet examined is totally absent in 
Holothurians. 
In the fully-formed Auricularia (fig. 1) the hydroccel, which 
is flattened dorso-ventrally, lies with its posterior end slightly 
nearer the ventral surface than its anterior end. Its inner 
face (that nearest to the cesophagus) is strongly concave, while 
its outer face, from which spring the radial vessels and tenta- 
cles, is convex. 
The position of the water-tube, which enters on the inner 
