Structure of the Brain of Liniulus polyphemus. 31 



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 under- 

 neath. 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, which reaches to the pos- 

 terior 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 

 sections 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 understanding the geography 

 of this ganglion. 



The study of the brain, in sections 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 Liniulus, and compare it with the normal 

 Arthropod brain. 



The histological elements of the brain of Limulus are four 

 in number : — 1. Large ganglion-cells, filled densely with gra- 

 nules, 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 contents very homo- 

 geneous ; 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 

 network 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 masses are dark or greyish, the sub- 

 stance or fibres enclosing them being whitish by transmitted 

 light. The brain is enveloped by a thick perineurium, formed 



