308 



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, tliat, 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 neurilemma, 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 horizontally 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 under- 

 standing the geography of this ganglion. 



The study of two brains , sliced horizontally each into over fifty 

 sections, carefully mounted in consecutive order, finally enabled me to 

 arrive at a tolerably complete idea of the relations of parts, so that 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. I) liarge ganglion cells, filled densely with granules and with 

 a Avell-defined nucleus similarly filled and with a granular nucleolus. 

 These cells may be crowded or loose, with the granules fewer in num- 

 ber, 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 contents very homogeneous, and they closely resemble 

 the nerve fibres distributed to the compound and simple eyes. Certain 

 fibres near the origin of the optic nerves , are distinctly nucleated at 

 intervals. 4) Rounded masses, consisting wholly of nuclei, enclosed in 

 a net work of fibres, which stain dark brown with osmic acid ; these 

 bodies form the larger part of the substance of the brain, while staining 

 dark brown with osmic acid; in unstained alcoholic sections these 



