130 HENRY BURY. 
organ” of Echinoderms with the “ pericardium” and “‘ pro- 
boscis gland ” of Enteropneusta respectively.! It is true that 
the observed position of the dorsal sac does not obviously 
accord with this, but I have endeavoured to show in the fore- 
going pages what its probable position was in the bilateral 
ancestor, and that position accords very well with the homology 
here suggested. 
Our present knowledge of the blood-vascular system in 
Echinoderms is too imperfect to allow of a detailed comparison 
with that of Enteropneusta, though there is much in the con- 
flicting evidence on the subject which is very suggestive. We 
may say, however, without much fear of contradiction, that the 
blood-vessels are simply lacunz, with no epithelial walls of 
their own, and that these lacune penetrate all through the 
complex structure of the “ dorsal organ ”—indeed in Holothu- 
rians, where the latter is absent, the lacunz alone are left in 
the place which it usually occupies (10). 
Again, in Echinid Plutei I have shown that the observed 
pulsation probably occurs in the raised mass of gelatinous 
tissue which projects into the floor of the dorsal sac, which 
gives this mass its only epithelial wall. Now this gelatinous 
tissue, though not so represented in my drawings (special stain- 
ing being required to demonstrate it) consists simply of a net- 
work of protoplasmic threads, the interstices of which (filled 
with a watery fluid) may be considered as parts of the 
original segmentation cavity. 
Without further discussion of the very obvious inferences to 
be drawn from these facts, and the equally obvious gaps in the 
evidence, I think I may claim to have established a case in 
favour of the homology of the dorsal sac and dorsal organ of 
Echinoderms with the pericardium and proboscis gland of 
Enteropneusta which cannot be lightly set aside; and taking 
these in connection with the other resemblances between the 
1 The former is apparently suggested by Morgan (22, p. 442), though for 
“ Auricularia’’ we must read “ Bipinnaria,” while the latter is suggested by 
Koehler, on the ground of similarity of structure; but neither offer much 
evidence, 
