66 HENRY BURY. 
same as in Brachiolaria (belonging to the second type of 
Asterid development), as shown in fig. 17. Dorsally and 
ventrally the left posterior body-cavity curves forwards in 
two horns, which, as it were, embrace the hydrocel, both 
horns being much longer than in fig. 17; the dorsal horn 
extends nearly to the water-pore, at the junction of stomach 
and oesophagus, while the ventral one runs alongside the in- 
testine, and almost at the level of the anus curves over 
(between the stomach and intestine), and fuses with the right 
body-cavity. This fusion makes it impossible to determine 
the exact limits of the two cavities (right and left), but we shall 
probably not fall into any important error if we regard the 
point of junction as median. 
Dorsally the right and left cavities already form a me- 
sentery of no great thickness; but ventrally they are still 
widely separated over the intestine, except at its extreme 
posterior end. 
The water-pore is, as usual, situated at the junction of 
cesophagus and stomach, rather to the left of the dorsal middle 
line. Slightly in front of the pore and in the median plane a 
cavity of irregular shape, absolutely unconnected with the 
body-cavities, lies over the base of the esophagus. I men- 
tioned it in a previous paper (9, p. 31) as probably of schizoceel 
origin, and this is rendered the more probable by the fact that 
the apparently homologous cavity in Asterias rubens and in 
Echinid Plutei arises in this way. In the present paper I 
propose to refer to it as the “ dorsal sac.” 
The larva just described measured about 1:7 mm. in length. 
The next larva obtained measured nearly 4 mm. (exclusive of 
the arms), and is shown in ventral view in fig. 18. In it the 
posterior end of the hydroccel is completely shut off from the 
body-cavity, and its anterior end, where it joins the anterior 
enteroceel, is further constricted. The dorsal horn of the left 
posterior cavity has fused with the anterior enterocel from the 
pore to the commencement of the hydroceel ; and the right 
cavity extends across the anterior end of the stomach, on the 
dorsal side, rather to the left of the middle line, so that the 
