116 HENRY BURY. 
connection of the water-pore with one of the plates of the left 
body-cavity (the so-called “ orals’”’). This pore was originally 
situated between the two series of skeletal plates, as ontogeny 
shows, and its connection with one or other of the plates adjoin- 
ing it has apparently occurred independently in the different 
classes; but in no case, to the best of my belief, has it ever 
wandered far away from the original line of separa- 
tion of the two body-cavities. 
The freedom of the intestinal mesentery to move where it is 
required has led in some cases to a portion of it traversing 
part of the right body-cavity. In my hypothetical ancestor 
the anus is placed in interradius A, at the level of the division 
of the body-cavities, as we find it in the larva of Antedon. In 
Asterids and young Echinids, however, we see that it has shifted 
into interradius E,and into the region of the right body-cavity— 
being aboral to the basal plates. This, however, may be due 
not toa shortening but to a lengthening of the intestine, 
which, in Kchinids at any rate (and in the larve of Asterids) 
reaches interradius A, and then (in Echinids) turns back to 
the anus in interradius E. 
It is just possible that this position of the anus may be 
primitive ; that at the very time that the mouth was working 
its way into the left side, the anus may have been moving into 
the right, but being (conceivably) prevented from reaching the 
pole by the dorso-central plate, it turned a little to the side into 
interradius E. In this case the position observed in Antedon must 
be regarded as secondary, due to a shortening of the intestine 
consequent upon the occupation of the aboral pole by the disc of 
fixation ; and the “ aboral longitudinal mesentery,”’ found no- 
where but in the larva of Antedon, and running from the aboral 
pole in interradius E to the level of the transverse mesentery in 
interradius A, very possibly marks the original course of the 
terminal portion of the intestine. I put this suggestion for- 
ward for what it is worth, knowing full well the weakness of 
the evidence. The ontogeny of other Crinoids may show us 
its true value, but the absence of the intestine in the larva of 
Antedon deprives it of all importance in this connection. 
