BRAIN AND CRANIAL NERVES OF BDELLOSTOMA DOMBEYI. 148 
Width. A B C D 
mm, nm. mim. mi. 
Olfactory lobes (middle) » eS 73 6-0 6:2 
Fore brain (cephalic end) . 45 65 5:0 53 
Fore brain (middle) . gh 0) 6°5 5D 56 
Mid brain (cephalic end) And 55 Ard 4-6 
Cerebellum __,, a 3° 35 2°6 3°0 
Medulla (through N. V.) 32 5D 50 50 
Medulla (through N. X.) £0 And 4-0 45 
Spinal cord : 2°2 2°5 2°0 2°5 
The brain is bilaterally symmetrical in every respect, and 
when viewed dorsally appears to be divided into main parts 
—a fore part, consisting of four sets of paired lobes, with a 
smaller median lobe, situated between the second and third 
pairs; and a hind part, an unpaired, shield-shaped division, 
with a V-shaped cleft cut into its cephalo-dorsal surface, in 
which cleft the posterior pair of lobes belonging to the fore 
part rest (fig. 1). The four sets of paired lobes proceeding 
cephalo-caudad are the olfactory lobes, fore brain, mid brain, 
and cerebellum, and the small median lobe lying between 
them consists of the habenular ganglia. ‘The large, unpaired 
hind portion is the medulla. 
In a view of the ventral surface these same divisions 
appear, but are of somewhat different form (fig. 2). The 
olfactory lobes alone appear in full size here. Slightly 
caudad of them the ’tween brain is to be seen, forming the 
most ventral prolongation of the brain. This important lobe 
extends caudad as far as the hind end of the mid brain, and 
laterad about half way from the median line to the lateral 
face of the brain, thus hiding from view the mesial portions 
of fore brain and mid brain. Close behind the caudal border 
of the ’tween brain, lies the cephalic border of the medulla. 
The medulla is ventral to almost the whole of the cerebellum, 
only the extreme cephalo-lateral parts of that section showing 
above it (fig. 2). 
The Olfactory Lobes,—The two olfactory lobes are, 
