BRAIN AND CRANIAL NERVES OF BDELLOSTOMA DOMBEYI. 163 
the optic is the smallest, and the acustic may be divided 
into four parts, two lateral line nerves, acusticus a and 
acusticus b, and two ear nerves proper, acusticus ¢ 
and acusticusd. There is no trace in the adult of any of 
the eye muscle nerves. 
N. Olfactorius.—Before describing the olfactory nerve 
it is necessary to give some idea of the organ innervated by 
it. The nasal organ of Bdellostoma is about 8 mm. long, 
7 mm. wide at cephalic end, and 9 mm. wide at its caudal 
end,and 7 mm. deep. The measurements belong to specimen 
B of the previous table. The organ is bilaterally sym- 
metrical, and consists of seven trapezoidal folds, hanging by 
their broader bases to the dorsal wall of the nasal capsule, 
and tapering to a point at the cephalo-dorsal surface. The 
median fold expands into a bulb at the cephalic end. Besides 
these seven free-hanging folds there are two smaller half 
folds, or flaps, fastened throughout almost their whole extent 
to the lateral walls of the capsule. Each half of the median 
fold belongs physiologically to its own side of the organ, and 
is innervated exclusively by the nerve of that side. These 
folds are related in a very definite way to the divisions 
marked on the surface of the olfactory lobes. ‘To make the 
description simpler I shall call the half of the median fold A, 
the other folds in order B, C, D, and the lateral flap E. The 
nerve-fibres of each of these folds gather into bundles as they 
leave them, one bundle for each half of a fold, and in this 
form pierce the cranial wall. Once within the cranium the 
bundles break up, the different sets of fibres separating to 
seek their respective endings in the olfactory lobes. The 
nerve-fibres of A and Brun exclusively to glomeruli in the 
dorsal part of the olfactory lobes, those of A to the inner 
division only, those of B to the inner and middle divisions. 
The C fibres run to both the dorsal and ventral parts of the 
lobe, the dorsal fibres being distributed to the middle 
division, the ventral fibres ending in all three, but princi- 
pally in the inner and lateral ones. The fibres of D and EK 
run exclusively to the ventral half of the lobes, those of D 
