410 
H. BUEY. 
left enterocoels and the hydrocoel; for though some observers 
asserted the presence in Ophiurid and Echinid Plutei of a 
second hydrocoel, yet the existence of this was not generally 
accepted. 
In my paper on Antedon (7) I showed that there exists in 
the larva of this animal a separate cavity (anterior body-cavity), 
median, or nearly so, with which the water-pore is related, and 
into which the hydrocoel subsequently opens by means of the 
water-tube (= stone-canal) ; and I pointed out the existence in 
Asterina gibbosa of an apparently homologous cavity. I 
shall now try to show that this cavity is always represented in 
Echinoderm larvae, but it is not always unpaired, as in Ante- 
don, a very distinct fellow to it on the right side being fre- 
quently visible. This condition, with two anterior enterocoels, 
I consider to be probably the most primitive, and I shall 
therefore begin my account with a description of those forms 
in which it obtains. 
Ophiurids. — The youngest Ophiurid Pluteus I was 
able to obtain had already a pair of cavities lying beside the 
oesophagus, but none as yet beside the stomach ; and was, in 
fact, in the stage described and figured by Metschnikoff (18, 
p. 21, pi. v, fig. 2). The only account we possess of the origin 
of these cavities is that of Apostolides (3), who believes that 
both in Ophiothrix versicolor and in Amphiura squa- 
mata they are formed by delamination in the mesoblast. 
Since, however, he describes the gastral cavity of these forms 
as also formed by delamination, while Kowalevsky (13, p. 5) in 
Ophiura (sp. ?), Selenka (27) in Ophioglypha lacertosa, 
Balfour (4) in Ophiothrix fragilis, and Fewkes (9) in 
Ophiopholis aculeata, find it to be formed by invagina- 
tion, we may be permitted to think either that Apostolides is 
mistaken, or that he has studied exceptional forms ; and that 
in some Ophiurids, at least, the two cavities beside the oeso- 
phagus arise as pouches of the archenteron, as in other Echino- 
derms. In any case the homology of these cavities with the 
similarly placed pair in Echinids and Asterids is hardly likely 
to be disputed. 
