AND EMBRYOLOGY OF LIMULUS. 35 



brain of the crayfish and lobster and insects is beautifully marked (each hemisphere 

 exactly repeating in its internal topography the structure of the opposite side), while 

 that of Limulus is obscure and imperfect. 



Stnicture of the oesojihageal ganglia. (Plate 6, fig. 11.) A section through one side of 

 the oesophageal ring, running through a ganglionic centre and the origin of the nerve to 

 one of the anterior (second?) gnathopods {gnn) shows that the topography is quite simple. 

 The central mass is mostly composed of nuclei and nerve fibres, the latter predominating 

 until- the nuclei disappear towards the base of the ganglion, where the nerve to the foot- 

 jaw originates. On the outside of the ganglion, along nearly the whole length, are 

 scattered large ganglion-cells {Igc). Near the upper and outer side is a group of small, 

 narrow nucleogenous bodies {nh). There is a wide space for the passage of the blood 

 between the ganglion with its nerve and the connective-tissue envelope, which is thick and 

 of the same structure as the perineurium of the bi*ain itself This space extends along 

 the whole length of the nervous system to the termination of the cord, the nerves sent to 

 the appendages being enveloped by a continuation of the same coat. Among the large 

 ganglion cells are numerous smaller ones, some of which are truly bipolar, as represented 

 in our drawing (fig. 11«) ; the nuclei have distinct edges, so that I regard them simplyas 

 small-sized ganglion-cells rather than nucleated nerve-fibres. 



Structure of the abdominal ganglia. There are six abdominal ganglia, the last being 

 larger and longer than the others. A section through the second abdominal ganglion 

 (plate 6, fig. 12) shows that the central mass of the doable ganglion consists of longitudinal 

 fibres, with scattered nuclei. On the upper side in the median line is a group of large and 

 small ganglion-cells, and beneath is a mass extending to each side where they become most 

 numerous. In some sections the central fil^rous mass is enveloped by an irregular layer 

 of ganglion-cells, some bipolar, with nerve-fibres forming a loose net work. In fig. 12 a 

 nerve connected by its neurilemma with that of the ganglion has been cut through ; in 

 this nerve there are only fibres present. In fig. 13 a large nerve leading to the 

 abdominal appendage is seen to be sent off from one side of the double central mass ; the 

 other side {gang) has been torn away from the one opposite. 



In neither this nor in sections of the last elongated abdominal ganglion were any 

 nucleogenous bodies to be seen, so it seems most probable that none occur in the 

 abdominal ganglia. 



The section here figured of the last abdominal ganglion (plate 6, fig. 14) is seen to pass 

 through four nerves, two on each side. The ganglion is seen to be formed by the union 

 of the two separate cords, which are separate just before the ganglion. Above the 

 ganglion on each side of the median line is a mass of large ganglion cells, of the same size 

 as those of the brain, associated with more numerous smaller ones. This mass extends 

 around and beneath each hemisphere of the ganglion, forming a layer of cells and fibres, 

 some of the cells distinctly bipolar, which becomes interrupted at the median line, indicated 

 by the deep notch in the central fibrous nerve-mass. The fibres from the laterally- 

 situated cells are distinctly seen passing in and mingling with the fibres of the central 

 nerve-mass; thus the nerves are reinforced from the peripheral ganglion cells. The 



