32 On the Brain o/'Limulus polyphemus. 



of a fibrous tissue and some (probably) elastic tissue, which 

 occasionally penetrates into the brain-substance between the 

 white rounded fungoid masses, forming the meshwork sur- 

 rounding them. The general topography of the brain of 

 Limulus is on a simple plan compared with that of Deca- 

 podous 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 centre of 

 the primary masses is stained paler brown by osmic acid. 

 These bodies are often seen in section rounded, but more often 

 are irregular, not closed, spheroids ; these dark bodies extend 

 through the brain like ruffles. The lower half or two thirds 

 of the entire brain is apparently filled with these nucleogenous 

 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 pre- 

 serve 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 

 becomes 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 to- 

 wards the origin of the nerves, as if they were the roots, as they 

 undoubtedly are. In the section passing through the ocellar 

 nerve and the tegumentary nerves on each side, the nucleoge- 

 nous 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 the nucleogenous 

 bodies. 



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

 in an indifferent state *, and the dynamic part confined to 

 the upper third, the region giving origin to the nerves of 

 sensation. 



* This area, made up of granules and nuclei, seems really to be con- 

 nective tissue, and to represent the connective tissue in which the ganglia 

 of the embryo of the young larva are imbedded. There seems no reason 

 why the brain should not be partly formed from connective tissue as 

 much as the remaining ganglia, as we have seen them to be in different 

 sections of different ganglia, all or nearly all except the supraoesopha- 

 geal one. 



