BRAIN AND CRANIAL NERVES OF BDELLOSTOMA DOMBEYI. 153 
would bring the saccus to the position it actually does occupy 
in higher forms. 
The Ventricles of the ’Tween Brain.—These are 
of the usual type in Bdellostoma, but differ somewhat from 
those of higher forms in some of the details. The iter, 
running directly ventral to the habenular ganglia, is always 
small, its diameter varying in different specimens. In some 
the lumen, though contracted, can be traced unbroken into 
the third ventricle, in others the lumen is entirely closed 
shortly after the entrance of the iter into the ’tween brain, 
if not, indeed, before, but its course may still be traced by 
an irregular column of ependyma cells. In other brains still 
even the ependyma cells are scattered or absent. The third 
ventricle, though small, is always open; it is situated a little 
eaudad of the anterior end of the right habenular ganglion, 
and is bounded as usual by the lamina terminalis. It is 
very much smaller in the adult than itisinthe embryo. The 
infundibulum arises from what appears in the adult to be the 
cephalic end of the iter, but what the embryo shows to be 
in reality the posterior part of the third ventricle. In the 
adult the dorsal part of the infundibulum is always completely 
closed, and its presence indicated only by ependyma cells. 
In some specimens these cells are arranged in separated 
groups, and are hard to find; in others they are grouped 
compactly together, and their line may be followed with 
comparative ease. A little more than half-way between the 
third ventricle and the ventral surface of the ’tween brain, 
the infundibulum takes definite form. It is a long, narrow 
slit, lying in the median plane, showing longer in horizontal 
sections through its ventral end, than in those through its 
dorsal end, and directed caudo-ventrad (figs. 5, 18). In 
sagittal sections it is seen to be somewhat boot-shaped. The 
lumen becomes wider as the ventral surface of the ’tween 
brain is approached. At the anterior end of the most ventral 
level of this slit-shaped lumen is given off a small, unpaired, 
median recess directed forward. The hind end of the infun- 
dibular cavity is inserted into the middle point of a semi- 
