446 General Notes. [June, 
by a continuation of this membrane, as several longitudinal sec- 
tions of these nerves have taught us. The brain in a Limulus 
ten inches long, exclusive of the caudal spine, is about five or 
six millimetres in diameter ; it is flattened slightly above, and on 
the upper side has a shallow median furrow, indicating that it is 
a double ganglion. Three pairs of nerves and a median unpaired 
one (the ocellar), arise from the upper third of the anterior face of 
the brain. The two optic nerves are the largest ones, arising one 
on each side of the median furrow, so that the fifth to fifteenth 
sections made by the microtome, pass through them. Next below 
(from above downwards) is the origin of the ocellar nerve, which, 
as described by A. Milne Edwards, is single, arising from the 
median line; on each side and in nearly the same plane, arise two 
tegumental nerves, and directly below a second pair of larger 
nerves (fronto-inferior tegumental) descend vertically. No nerves 
arise from the lower half or two-thirds of the brain, which is 
smooth and rounded, with no median furrow underneath. It will 
thus be seen, that, as stated by A. Milne Edwards, there are no 
antennal nerves, such as exist as a rule in Arthropods except 
Arachnida. This we have proved in the same manner as Milne 
Edwards, by laying open the arterial coat or modified neurilem- 
ma, which reaches to the posterior end of the brain, and seeing 
that the fibres of the nerves sent to the first pair of legs originate 
quite independently of the brain itself. 
Internal Structure and Histology of the Brain—Transverse sec- 
tions of the brain throw but little light on the topography, as the 
nerve fibres extend horizontally, the nerves being sent out horizon- 
tally and from the anterior end only of the brain; hence the 
examination of nearly two hundred sections threw little light on 
the topography, and considerable time was spent in a vain and 
baffling attempt at understanding the geography of this ganglion. 
The study of two brains each sliced horizontally into about fifty 
sections, carefully mounted by Mr. Mason in consecutive order, 
finally enabled me to arrive at a tolerably complete idea of the rela- 
tions of parts, sothat I could mentally construct a model of the brain 
of Limulus, and compare it with the normal Arthropod brain. | 
The histological elements of the brain of Limulus are three in 
number. 1. Large ganglion cells, filled densely with granules and 
with a well-defined nucleus similarly filled and with a granular 
nucleolus. These cells may be crowded or loose, with the granules 
fewer in number, and with loose, thick cell-walls; they terminate 
in large fibres which sub-divide. 2. Similar cells, but smaller 
with less protoplasm, and like those in the lobster’s brain. 
3. Nerve fibres; these, like the large sized ganglion cells, from 
which they originate, are stained tawny yellowish-brown with 
osmic acid. These fibres are large, coarse, their granular con- 
tents very homogeneous, and they closely resemble the nerve 
fibres distributed to the compound and simple eyes. Cem 
fibres near the origin of the optic nerves, are distinctly nucleate 
