BRAIN AND CRANIAL NERVES OF BDELLOSTOMA DOMBEYI. 145 
together, and lie with their axes arranged more or less 
longitudinally. Aside from the glomerular layer, the 
olfactory lobes consist of a loose mass of cells and fibres. 
The cells are, roughly speaking, of two kinds :—comparatively 
large multipolar cells, staining lightly in hematoxylin, and 
having large, round or ovoid nuclei, and smaller cells, spindle 
or multipolar, that stain an intense blue black. They are 
not arranged in any such definite way as the cells in other 
parts of the brain. The fibres are the secondary olfactory 
fibres. ‘They come from all parts of the lobe, gathering into 
tracts as they leave it. The two chief tracts are the tractus 
olfacto-habenularis, and the tractus olfacto-cere- 
bellaris. The tractus olfacto-habenularis leaves the 
olfactory lobe at the median dorsal angle and goes directly 
to the habenular ganglion, without any way station in the 
fore brain as is usual in the higher forms. The tractus 
olfacto-cerebellaris leaves each lobe at about the middle 
of the caudo-ventral border, and courses back, through the 
floor of the ’tween brain, ascending finally to the roof of the 
cerebellum. 
The fibres of the olfactory nerves enter each olfactory lobe 
in the dorsal and lateral parts of its cephalic face. Owing to 
the fact that the nasal organ les immediately adjacent to 
the cranium, and covers its anterior end wail, its nerves have 
neither the need nor the opportunity of uniting into one 
great nerve-trunk, as in other forms, but pass through the 
cranium and enter the olfactory lobes in the same bundles in 
which they leave the nasal organ. 
The Fore Brain.—The fore brain, as mentioned above, 
consists of two lobes, a right and a left, equal and sym- 
metrical. They are slightly smaller than the olfactory lobes, 
narrower across the caudal than across the cephalic face, and 
are separated cephalo-dorsad by a deep cleft, the contination 
of the cleft between the olfactory lobes. In the median 
caudal part they fall away from each other, admitting 
between them the dorsal part of the ’tween brain, the 
habenular ganglia, ‘he dorsal cleft between them extends 
