——ae 
SENSE ORGANS OF AMIURUS. 353 
therefore devoted to the origin and distribution of the trigeminus 
group and to the auditory apparatus. The following order is observed 
in the description of the various parts :— 
I. Central nervous system. 
II, Peripheral nervous system. 
III, Sense organs. 
I. CENTRAL NERVOUS SYSTEM, 
A.—THE BRAIN. 
As in most other Teleosts the cranial cavity of Amiuwrus is by no 
means filled up by the brain, which is surrounded by a large quan- 
tity of areolar connective tissue rich in vessels and fat. This tissue is 
continued backwards into the neural canal and into the cavities in 
which the semicircular canals are lodged, to which, and indeed to the 
whole auditory labyrinth, the tissue acts as ‘ perilymph.’ 
The recent observations of Mayser! and Rabl-Riickhard? have con- 
firmed Stieda’s interpretation of the various parts of the Teleost 
brain, and are thus entirely opposed to the views expressed by 
Fritsch in his “ Untersuchungen tiber den feineren Bau des Fisch- 
gehirns.” As was to be expected from the affinity of the Siluroids 
to the Cyprinoids, I have found Mayser’s researches, which are 
chiefly based on the latter group, of the greatest service in studying 
the brain of Amiurus. The points in which that genus differs from 
Cyprinus I shall call attention to in the course of my description 
My observations have, however, not been extended to the study of 
the finer structure of the brain, and the sections figured are rather 
intended to complete the topographical description than to furnish an 
exhaustive account of the nerve-fibre tracts. 
Owing to the abundant perilymphatic tissue it is easy to remove 
the roof of the brain case without injuring the brain. The appear- 
ance of the organ when so exposed is represented in Fig. 13, Pl. I. 
In front we have the so-called cerebral hemispheres (CH) which atter 
the brain has been hardened appear to be two solid masses separated 
by a longitudinal medial groove, but which in the recent condition 
are seen to be two oval thickenings in the floor of a sac whose roof 
and walls are extremely thin and transparent, and whose cavity is the 
ventriculus communis of the secondary forebrain, prosencephalon. In 
comparison with many other Teleostean forms the cerebral hemis- 
pheres of Amiwrus are of large size. From the ventral surface of 
1 Zeit. wiss. Zool. XXXVI. 2 Arch. Anat. Phys. 1882-3. , 
