8o MINUTE STRUCTURE OF THE BRAIN AND CORD OF ARACHNIDS. 



The principal centers are : a. a large oval center at the root of the pedal nerve, 

 forming the medullary core to the pedal ganglion. (Fig. 60.) A prolongation of 

 it extends cephalad and mesad, forming a compact, oblong mass, near the center 

 of the neural surface of the neuromeres; n.p. b. A center for the cephalic root 

 of the hsemal nerve, Hr', extending along the median neural surface of each 

 ganglion, c. A center for the middle root of the haemal nerve, Hr^ on the ante- 

 rior median face of the neuromere. d. The haemal tracts; large, spindle-shaped 

 tracts, one on either side of the median line, on the haemal surface. (Figs. 62, 67, 

 68, l.h.tr.) (e) Four small masses, on the hjemal surface, between the anterior 

 and posterior haemal commissures and the longitudinal haemal tracts. For longi- 

 tudinal tracts in sections, see Figs. 67 and 68. 



II. The Cephalic Neuromeres. 



We are now in a position to describe the arrangement of cells and fibers in 

 the cephalic neuromeres. 



The brain neuromeres, in the main, closely resemble those of the cord. 

 The principal differences in form are due to their linear union and to the lateral 

 divergence of the crura. The histological differences are due mainly to the 

 absence of motor neurones such as the hypobranchial, intestinal, and cardiacs, to 

 the greater size and isolation of the pedal ganglia, and to the presence of the 

 large gustatory nerves. 



Cell Clusters. — The nerve cells are arranged in clusters, of varying sizes, 

 that have special neurilemma sheaths, as in the cord, but they are so crowded 

 together that it is difficult to determine the exact arrangement. They cover the 

 neural surface and lateral margin of the crura, leaving the commissures, part of 

 the gustatory tracts, and the haemal surface exposed. (Figs. 65 and 66.) 



The Commissures. — Each neuromere has several bundles of cross commis- 

 sures that have terminal relations similar to those described for the branchial neu- 

 romeres. Owing to the divergence of the crura they form long, backwardly directed 

 loops in which the commissural fascicles are difficult to identify, except where 

 they approach the crura. 



In very young crabs, sagittal sections show that the commissures of each 

 neuromere are surrounded by distinct membranes. There are two groups of 

 commissures for each neuromere, corresponding to the neural and haemal commis- 

 sures of the cord, and no doubt containing similar components. In the adult, 

 the median portion of the anterior thoracic commissures form compact bundles 

 with a common neurilemma sheath; near the crus, the several fascicles separate 

 to their respective terminals. (Fig. 56.) The more posterior thoracic commis- 

 sures, and those in the hindbrain, are shorter, and the neural and haemal fascicles 

 are widely separated, leaving a space between them, which represents the begin- 

 ning of the fourth ventricle. (Figs. 46, 47 and 55.) 



The neural commissures. — I have not been able to work out the relation 



