96 HENRY BURY. 
The Bilateral Ancestor. 
Into the most primitive stages of the bilateral ancestor I do 
not propose to enter here; some few remarks on the subject 
will be offered at the end of this paper, when I come to discuss 
the relation of the Echinodermata with the Enteropneusta, 
but for present purposes it will be enough to start with a stage 
—of the existence of which I believe there is sufficient evidence— 
in which the hydrocel already formed a ring round the 
cesophagus, and five tentacles already laid the foundation of the 
future pentamerous symmetry, while behind the cesophagus 
the alimentary canal and body-cavities still retained the 
primitive bilateral symmetry. Such a form is shown in figs. 
44 and 45, and may be described as follows: —The mouth was 
bent down on to the ventral surface, and probably opened into 
an atrial cavity, though this is uncertain; the stomach was 
globular or slightly elongated antero-posteriorly, and from its 
posterior end the intestine ran forward, opening on the ventral 
surface not far from the level of the anterior margin of the 
stomach. Round the esophagus was the water-vascular ring, 
with five tentacles or tube-feet, suitable to progression, but 
capable of retraction within the atrial cavity (if that existed at 
this stage). One tentacle was median and posterior, while in 
the dorsal interradius, on the left side of it, was the water-tube 
running back to the water-pore, which was situated over the 
anterior end of the stomach, rather to the left of the middle 
line. The left anterior enteroccel was probably already reduced 
toasimple ampulla at the junction of water-tube and pore- 
canal, such as we find in adult Echinoderms, though it probably 
ran a little forward (see fig. 45) alongside and on the dorsal 
side of the water-tube. The right anterior enterocel had 
probably already disappeared. The two large body-cavities 
were symmetrically disposed on the right and left sides of the 
stomach, and there was certainly between them a longitudinal 
dorsal mesentery, though how far this extended round to the 
ventral surface is uncertain. In the middle line, just to the 
right of the water-pore, I believe there was a “ dorsal sac,” 
