76 MINUTE STRUCTURE OF THE BRAIN AND CORD OF ARACHNIDS. 



(Figs. 60, 61, 63, 64, F.) Their axones form a rather loose bundle (ill defined in 

 sections) extending downward, inward, and forward along the neural portion 

 of the cord, to the next anterior neuromeres. Before and after crossing, the large 

 irregular fibers give off numerous collaterals which ramify diffusely in the central 

 fibrous portions of the neuromere. (Fig. 63, i*'.) Termination unknown; probably 

 association fibers. 



g. On the anterior lateral part of the neural surface, small clusters not 

 clearly defined, that send axones diagonally inward and forward. (Figs. 61 and 

 63, G.) The scattering axones appear to cross in both the anterior neural and 

 the anterior haemal commissures. Some appear to send collaterals backward to 

 the next posterior neuromere. Termination unknown. 



h. Along the lateral haemal margin, on either side of the pedal ganglion, 

 are several groups of cells that are usually very conspicuous. (Fig. 62, H''"''^.) 

 They differ from the other neurones in that each cell gives rise to a large number 

 of dendrites and axones. The dendrites are minute and their innumerable 

 branches fill the core of the pedal ganglion, often giving it a dark blue, finely 

 granular appearance. The axones are large, irregularly branching fibers ex- 

 tending outward, as bundles of parallel fibers, onto the haemal surface of the pedal 

 nerve. These neurones are the only ones of their kind and are characteristic of 

 the pedal nerves, both in the thorax and in the abdomen. 



The axone bundles from the several clusters, iJ'~^, converge toward the 

 haemal side of the pedal nerve where they form a distinct bundle, readily recog- 

 nizable in sections. (Fig. 68, h.) They are the motor nerves that supply the gill 

 muscles. 



i. Two large groups of neurites on the anterior hjemo-lateral margin, sending 

 great bundles of fibers forward and inward into the anterior portion of the an- 

 terior haemal commissure. (Fig. 62, 1 and 64,^.) 



j. A large group of cells on the posterior haemal margin, projecting their 

 fibers forward and then across to the opposite side, in the anterior portion of the 

 posterior haemal commissure. (Figs. 62, and 68, J, von Rath's preparations.) The 

 cells and fibers of this group have not been identified by the methylene blue 

 process. 



NERVE-ROOTS. 



The Neural Roots. — The neural or branchial nerve arises from a large 

 ganglion on the posterior neuro-lateral surface of the neuromere, and extends 

 upward (neurally) and outward to the gill. In cross sections (von Rath's 

 preparations) near the neuromere, it consists of two portions, the larger one 

 formed of a coarse polygonal meshwork of neuroglia, each mesh crowded with 

 black dots, representing the cut ends of innumerable nerve fibers. (Fig. 62, s.t.) 

 These sensory fibers constitute about three-quarters of the entire nerve. Most 

 of them terminate in very fine dendrites, in the large oval mass of neuropile that 



