THE CEPHALIC NEUROMERES. »I 



of all the commissural elements in detail, although some of them stand out very 

 clearly. For example, in methylene blue, the posterior neural commissures are 

 often very conspicuous. They extend diagonally forward (anterior ones) and 

 outward on to the neural surface of the crura, over the great gustatory tracts, to 

 the cell clusters E, on the posterior haemo-lateral margin of its neuromere. (Fig- 

 65, E, p.n.co.) 



A bundle of these fibers, on the anterior side of the second thoracic neuro- 

 mere, indicates that the cheliceral neuromere, in spite of its distinctly pre-oral 

 position, has its commissure behind the oesophagus. In most cases, one may 

 recognize two sets of these neural fibers to each neuromere, an anterior one, 

 arising from the neurones E, and a posterior fascicle, ending in a separate mass of 

 neuropile on the lateral half of each crus. 



The hsemal commissures contain several sets of fibers. The ones most 

 clearly seen in methylene blue preparations are rather large fibers which arise from 

 neurones A (Fig. 66) and after crossing, enter the roots of the hasmal nerves. 



Between the nerve roots and the commissure, are two sets of longitudinal 

 fibers extending outward and backward along the haemal surface of each crus, 

 one on the median side, one on the lateral. (Fig. 66, left.) Most of the longi- 

 tudinal fibers come from the opposite side through the haemal commissure; some 

 of the lateral ones probably come from neurites, B, on the neural surface of the 

 same side, corresponding with group B of the cord. 



The Haemal Nerve Roots. — It will be recalled that the cranial hasmal 

 nerves supply the integument of the cephalothorax. The branches, which 

 in the abdominal nerves run to the great longitudinal muscles, to the branchio- 

 thoracic muscles, to the heart and to the intestines, are absent from the six pairs 

 of thoracic, hasmal nerves. Hence they have but a single root, mainly, if not wholly 

 sensory, and representing root two, H.r\ of the branchial neuromeres. 



In most preparations, the hasmal roots appear to extend only part way through 

 the crus, terminating abruptly in the main longitudinal tracts on the median side 

 (Fig. 66, right side) . They appear to end there in a mass of neuropile, like that 

 of the second root of the abdominal nerves, H.r"^. In other preparations, many 

 fibers are seen to enter the haemal commissure and terminate in the A neurones 

 of the opposite side, which no doubt correspond to the A neurones of the abdomen. 



No trace of any other roots could be found. From this observation we may 

 infer that roots two and three, H.r'', and H.r^, of the abdominal hsemal nerves 

 are sensory general cutaneous; that root one, H.r^, contains the cardiac elements; 

 and that the neurones, D, are distributed to the branchio-thoracic muscles. 



The Neural Nerve Roots and the Cranial Ganglia. — Owing to the greater 

 size and speciahzation of their terminal organs, the neural, or pedal nerves of the 

 head are much larger and more complex than those of the branchial region, but 

 in the minute structure of their ganglia and nerve roots they are much alike. 



Ganglia. — In the adult Limulus (Fig. 218), each cranial ganglion forms a 

 large oval mass of neuropile, projecting a considerable distance from the sides 



