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FISHES CHAP. 
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| 
In the Cyclostome Petromyzon there is a small prosencephalon 
with an undivided prosocoele, and on each side of it a small 
cerebral hemisphere which appears as a mere appendage to the 
much larger olfactory lobe (Fig. 211). Thé prosocoele divides 
in front into two outwardly directed branches, and of the two 
diverticula into which each branch divides one extends as a 
lateral ventricle into the hemisphere 
of its side, and the other as a rhino- 
coele into the corresponding olfactory 
lobe. The gangha habenulae are 
unusually large, the right one being 
larger than the left. The optic 
lobes are large, but not obviously 
double. So small is the cerebellum 
that it seems to be little more than 
a narrow transverse band crossing 
the fore-part of the fourth ventricle. 
The roof of the brain is largely 
epithelial, especially in the prosen- 
| cephalon, the thalamencephalon, and 
Fra, 212.—Dorsal view of the brain the hind-brain. Over these epi- 
of Myxine. ¢.r, Corpora resti- . [ : : 
formia ; m.o, medulla oblongata; thelial areas the pia mater is un- 
mp, naso-pituitary canal; ol.0, usually vascular and forms a series 
olfactory organ enclosed in its 5 a ” .* 
fenestrated cartilaginous capsule ; of “ choroid plexuses. The ventri- 
op.l, optic lobes; pr, prosen- cylar system is complete and con- 
cephalon ; s, s, dorsal roots of . : 
spinal nerves ; sp.c, spinal cord; tinuous. By contrast with the 
Wiedesice shes Heizwag'™™ Lamproy the brain of Mynine* i 
very primitive, more so _ perhaps 
than in any other Craniate (Fig. 212). In a dorsal view the 
brain is divided into four pairs of laterally expanded and longi- 
tudinally compressed lobes by a median longitudinal fissure and 
three transverse fissures. The two anterior lobes are little more 
than the thickened anterior wall of the thalamencephalon, 
although, judging from their histological structure, they represent a 
very imperfectly differentiated prosencephalon and olfactory lobes. 
The second and larggst pair constitute the thalamencephalon. 
The last two pairs of lobes represent a transversely divided pair 
of optic lobes, or “corpora quadrigemina.” There is a large 
medulla oblongata with a pair of corpora restiformia, but the 
1 Holm, Morph. Jahrb. xxix. 1901, p. 365, 
