BRAIN AND CRANIAL NERVES OF BDELLOSTOMA DOMBEYI. 149 
penetrate among the cells of the mid brain tectum, at about 
the centre of its lateral surface. The bundles of Meynert 
connect the habenular ganglia with the medulla. The 
bundles are symmetrically placed, but unequally developed, 
the right bundle measuring about ‘28 mm. by ‘185 mm., the 
left 196 mm. by -12 mm. Their fibres arise mainly in the 
dorsal part of the ganglia,and gather in one common median 
strand at the cephalic end of the right ganglion. They run 
ventrad until they reach the ventral level of the ganglia, 
then, turning sharply, they run caudad, still as one common 
strand, in the median line, directly ventral to the ganglia. 
While on this course the strand is increased by fibres coming 
from the more ventral part of the right ganglion. At the 
posterior end of the right ganglion the bundles separate and 
turn caudo-ventrad, running to the base of the mid brain. 
Here they decussate for the first time, and pass on into the 
medulla, decussating again at the level of the entrance into 
the medulla of the fibres of ramus ophthalmicus V, the 
decussation stopping about ‘12 mm. cephalad of the exit of 
the motor trigeminus fibres from the medulla. 
The Thalamus.—The thalamus of Bdellostoma possesses 
no features of marked interest from the standpoint of gross 
anatomy. 
The Hypothalamus.—The base of the ’tween brain is 
rather a passage way for tracts than a place of origin and 
ending. The principal tract ending there is the one des- 
cribed above coming from the fore brain, and the crossing of 
whose fibres forms the great post-optic decussation. This 
tract and the tractus olfacto-cerebellaris occupy most 
of the floor of the ’tween brain. 
Anterior to the optic nerves there are, in the base of the 
*tween. brain, two small groups of cells, one on each side. 
These groups consist both of the dark and of the light cells, 
similar to those of the habenular ganglia, only larger. The 
cells in the caudal end of the ’tween brain floor, also of the 
same two kinds already described, are arranged somewhat in 
longitudinal strands, their long axes running athwart the 
