84 MINUTE STRUCTURE OF THE BRAIN AND CORD OF ARACHNIDS. 



of the brain. In the young (Figs. 35, 36, 38, 39), they are separated from the 

 brain by long narrow stalks, or tracts, devoid of neurones or neuropile. 



Most of the peripheral fibers terminate in the neuropile core, forming many 

 minute irregular centers (Fig. 65). There is a special neuropile center on the 

 anterior neuro-median side of each ganglion, formed by the terminals of a small 

 bundle of rather distinct fibers. This center in some cases appears like a dark 

 granular blotch, in others like a beautifully distinct, anastomosing network (Fig. 



65, w). Their peripheral relations were not determined. 



The nerve cells of the ganglia are very small and numerous. They are con- 

 fined in the main to the haemal surface, sending short, vertical axones into the 

 neuropile, where they divide, one branch passing outward into the nerve, the 

 other forming branches in the neuropile, which extend toward the crus. (Fig. 



66, C). 



The Motor Neurones. — On the haemal surface of the ganglia, there are two 

 very conspicuous bundles of coarse fibers, coming from large neurones on either 

 side of the ganglion. A third bundle joins them, coming from cells on the neural 

 side of the ganglion. They form the small anterior and posterior ento-coxal 

 nerves supply that the coxal muscles. (Fig. 66, a.en.cx and p.en.cx.) 



These neurones agree with the H neurones of the branchial ganglia, in that 

 each one sends a considerable number of axones into the nerve trunk. 



The Gustatory Nerves and Tracts. — The most striking features of the mid- 

 brain neuromeres are the large nerve roots supplying the taste organs in the coxal 

 spurs (Fig. 65, g.n.r}~^). They extend along the neural face of the pedal ganglia, 

 sweeping diagonally inward and forward, the inner end of each root overlapping 

 the next posterior one. The united bundles form an immense longitudinal tract 

 along the median neural margin of each crus, between the neural and heemal 

 commissures. 



The more posterior roots are the largest, that of the sixth appendage largest 

 of all. The deep, inner ends of the fifth and sixth roots form large oval neuropile 

 enlargements on the posterior median face of each crus. 



Toward the anterior end, one may recognize two subdivisions to the main 

 tract. Near the stomodaeal ganglion both divisions turn outward and downward, 

 then upward, forming a sharp semi-circular turn round the crus, giving the latter, 

 in cross-sections, a very unusual spiral structure. The larger bundle apparently 

 terminates in the great cheliceral lobe (Figs. 65 and 114, ch.l); the smaller one g.tr'^ 

 forms a slight dilatation, consisting of very dense nodular neuropile, on the lateral 

 margin of this lobe, and then passes straight forward, along the neural surface 

 of the cheliceral neuromere to the median cerebral lobe, G.ctr^. 



The very large fasciculus of the sixth nerve comes, not from the coxal spurs, 

 which here form crushing mandibles devoid of gustatory spines, but from the 

 large spatulate organ on the outer margin of the coxa (fiabellum) , the function of 

 which is to test the composition of the water passing to the gill chamber. 



