BRAIN AND CRANIAL NERVES OF BDELLOSTOMA DOMBRYI. 147 
will be necessary for the later stages in the development of 
the brain of Bdellostoma to be more thoroughly worked out 
before it will be possible to homologise the parts of the adult 
fore brain with those found in higher forms. 
The Tween Brain.—This important section of the brain 
is wedge-shaped, its broad flat base covering almost the whole 
under surface of the fore part of the brain, and its sides 
sloping inward as they rise, until they meet in a ridge in the 
habenular ganglia. The wedge is bilaterally symmetrical, 
and also leans somewhat cephalad, so that a sagittal median 
section is somewhat trapezoidal in shape, / /. The habenular 
ganglia are comparatively large, appearing externally as a 
separate division of the brain (see above). They lie, as before 
mentioned, close to the median line,—about midway between 
the lobes of the mid brain and those of the fore brain (fig. 1), 
but do not project as high as either of these; consequently 
they cannot be seen in a side view of the brain. They form 
the most anterior portion of the ’tween brain, projecting 
decidedly cephalad of the rest of this segment of the brain. 
In the specimen measured the right ganglion was 1:2 mm. 
long, ‘5 mm. wide, and ‘93 mm. deep; the left ganglion 
was ‘78 mm. long, ‘39 mm. wide, and *74 mm. deep, and 
was so placed that it extended -27 mm. cephalad of the 
right ganglion. Bdellostoma possesses no trace of an epi- 
physis. 
The base of the *tween brain extends nearly 1 mm. 
further ventrad than the surrounding parts of the brain, is 
shield-shaped in appearance, and slightly convex (fig. 2). 
It is marked by five small rounded hillocks. Through the 
first two of these, lying one on either side of the median 
line and about one third of the way back from the anterior 
edge of the ’tween brain, the optic nerves enter the brain. 
About half way between these and the hind end of the ’tween 
brain, situated in the median line, is the stalk of the infundi- 
bular process, and immediately behind it, one on each side, 
are two smaller projections, whose significance I have not 
been able to discover. The infundibular process is a small 
