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masses are dark or grayish, the substance or fibres enclosing them, 

 being whitish, by transmitted light. The brain is enveloped by a thick 

 perineurium, formed of a fibrous tissue, and some (probably) elastic 

 tissue, which occasionally penetrates into the brain-substance between 

 the white rounded fungoid masses, forming the mesh-work surrounding 

 them. The general topography of the brain of Limulus is on a simple 

 plan compared with that of Decapodous Crustacea and insects. The 

 brain is mostly composed of large irregular rounded masses or balls of 

 granules, with a thick fungoid or ruffle-like periphery, formed by a 

 layer of secondary smaller rounded granular masses. The center of the 

 primary masses is stained paler brown by osmio acid. These masses are 

 often, seen in section, rounded, but more often are irregular, not closed 

 spheroids, extending through the brain like ruffles. The lower half or 

 two-thirds of the entire brain is apparently filled with these nucleo- 

 genous bodies, as we may provisionally designate them. In the upper 

 third of the brain, whence the nerves originate, the larger ganglionic 

 cells and the nerve fibres appear, and preserve a definite topographical 

 relation to the entire brain. The nucleogenous bodies are confined at 

 the top to each side of the brain ; the central and hinder regions are 

 filled with the large ganglionic cells, mixed with numerous much 

 smaller ones, and the mass of nerve fibres which spring from them, be- 

 comes larger from the upper third to the top of the brain where the 

 optic fibres originate. Opposite the beginning of the optic nerves, 

 these large nerve fibres are seen directed towards the origin of the 

 nerves as if they were the roots, as they undoubtedly are. In the sec- 

 tion passing through the ocellar nerve and the tegumentary nerves on 

 each side, the nucleogenous bodies are situated in the front of the brain ; 

 but they disappear from the front higher up at the origin of the optic 

 nerves, and occupy a much more restricted area on the sides of the brain. 

 Thus the tract of nerve fibres on either side of the brain is irregularly 

 wedge-shaped, the apex situated near the centre of each hemisphere, 

 and the base spreading out on the top, thus crowding to the outer walls 

 tjie nucleogenous bodies. 



It would thus appear as if the lower half of the brain were in an 

 indifferent state ^, and that the dynamic part were confined to the upper 

 third, the region giving origin to the nerves of sensation. 



The asymmetry of the brain is remarkable ; the large ganglionic 

 cells are most abundant in the center behind the middle and from there 



1 This area, made up of granules and nuclei, seems really to be connective tissue, 

 and to represent the connective tissue in which the ganglia of the embryo, of the 

 young, larva are embedded. There seems no reason why the brain sliould not be 

 partly formed from connective tissue as much as the remaining ganglia, as we have 

 seen them to be in diiferent sections of different ganglia, all or nearly all except the 

 supra-oesophageal one. 



