THE CELL CLUSTERS OF THE BRANCHIAL NEUROMERES. 



73 



The posterior nerve is more voluminous, but less complex than the anteiior 

 one. It arises from a large conical "ganglion," consisting of neuropile and 

 ganglion cells, situated on the posterior neural surface of the cord. (Fig. 63, C.) 

 It supplies the gill muscles and the sense organs of the corresponding branchial 

 appendage. The majority of the elements are sensory, and are confined in the 

 main to the neural side of the nerve. It is therefore a neural ganghonated nerve, 

 comparable with the posterior, or ganglionated 

 root of a vertebrate cranial, or spinal nerve. 



Cell Clusters. — The nerve cells in the 

 neuromere are arranged in clusters that are 

 remarkably constant in their size, location, and 

 relation to fiber tracts. Each cluster probably 

 represents the remnants of one or more primitive 

 sense buds. 



In exceptional cases, the methylene blue fails 

 to affect the cells and fibers, but stains rather 

 sharply the neuroglia, thus giving excellent pic- 

 tures of the nerve cell clusters and their sheaths. 

 (Fig. 61.) The clusters are mainly confined to 

 the neural surface and lateral margins of the 

 neuromere. The more important ones are as 

 follows : 



a. A cluster of large cells in two or more 

 groups, on both neural and heemal sides of the 

 anterior lateral margin. (Fig. 61, A.) Their 

 axones cross to the opposite side, forming part of 

 the anterior haemal commissure, entering the 

 haemal nerve as its third root. (Figs. 61 h.r.^ and 

 62, a.) Before crossing, each axone gives off a 

 large collateral, a', that extends backward into 

 the longitudinal hjemal tracts. 



b. A very large cluster of medium size cells 

 (Fig. 61, B.), whose axones, b. converge into a 

 large bundle, directed vertically. On reaching 

 the haemal surface of the cord, each fiber divides, 

 one branch entering the main longitudinal haemal tracts and extending backward, 

 as a coarse unbranching fiber, to the more posterior neuromeres. (Fig. 62, b.); 

 the other branch, b', crosses to the opposite side, in the posterior part of the 

 anterior haemal commissure, behind the fibers forming the third root 

 of the haemal nerve. They are probably association elements. (Figs. 61, 62 

 and 64.) 



c. Numerous clusters of minute cells, on the neural surface of the pedal 

 ganglion. Their axones terminate in the large mass of interwoven fiber bundles 



Fig. 60. — Two branchial neuromeres, 

 seen from the neural surfaces, showing the 

 location of the principal masses of neu- 

 ropile, fiber bundles, and neurones. 



