120 HENRY BURY. 
In one of these two ways, it seems to me, we may find a 
possible explanation of the very curious course taken by the 
dorsal organ in Crinoids. So striking is the difference of 
position of this organ in Echinozoa and Pelmatozoa, that Cuénot 
denies the usually accepted homology. In combating this view 
MacBride (18, p. 147) suggests that the dorsal organ of Crinoids 
may be homologous with that portion of the “ovoid gland” 
in Asterids and Ophiurids which lies on the aboral side of the 
genital rachis. But this seems to me to imply misconception 
of the relations of the parts involved. The remarkable point 
about Crinoids is that the dorsal organ traverses the right 
body-cavity, no part of it, so far as we know at present, lying 
in the region of the left cavity. In Echinozoa, on the other 
hand, almost its entire course is in the region of the left cavity, 
the only portion which extends beyond this region being the 
small aboral portion which ends in the dorsal sac in Asterids 
and Kchinids, and lies under “ sinus b” in Amphiura (18, 
fig. 2,e). There is not the smallest evidence that the dorsal 
organ in any of the Echinozoa extends beyond the mesentery 
into the region of the right body-cavity. This, so far as it 
goes, furnishes an argument in Cuénot’s favour; but the 
other objections to his view are so many and important, that 
most embryologists will be loth to accept it, so long as any 
other explanation is possible. 
Now if, as I have supposed, the ancestor of the Pelmatozoa 
was fixed by a point lying over the dorsal organ, and 
this point afterwards migrated to the aboral pole, might 
not the subjacent organ share in this migration, and after- 
wards, taking the shortest course from this pole to the mesen- 
tery, pass, as it does in Antedon, through the right body-cavity, 
in the concavity of the alimentary canal ? 
Of course this is only put forward as a suggestion, and 
would require a great deal more evidence to prove it; but at 
least it seems to me to be more satisfactory than Cuénot’s 
denial of homology, which no amount of fresh evidence seems 
likely to make wholly satisfactory, placing, as it does, a wider 
gulf between the Echinozoa and Pelmatozoa than the many 
