72 MINUTE STRUCTURE OF THE BRAIN AND CORD OF ARACHNIDS. 



arachnids form the lateral walls of the neural tube of vertebrates, the middle cord 

 canal (canalis centralis) being at the inner surface, the neural crests on the outer 

 one, and the nerve fiber layers on the lateral wall of the tube. (Figs. 134-137.) 



Conunissures. — In arachnids, two main sets of transverse commissural 

 fibers are formed in each neuromere, the neural and the haemal commissures. 



The haemal commissures are the first to 

 arise. They make their appearance on the 

 haemal side of the epithelium of the median 

 groove as two separate bundles, an anterior 

 and a posterior one. (Fig. 64, a.h.co. and 

 p.h.co.) The haemal commissures are im- 

 portant features in all arthropod neuromeres, 

 and represent in part the remnants of the 

 transverse fiber tracts that extend round the 

 body and unite the longitudinal cords. Seen 

 in sagittal sections of the adult cord, they 

 appear as two large irregular bundles, 

 separated by a narrow space through which 

 the neuroglia of the median canal and me- 

 dian fissure is continuous with that of the 

 lemmatochord. 



At a much later period a new set of com- 

 missures appears above the floor of the median 

 groove, thus converting that part of the groove 

 into a canal; a large anterior one, an.n.co., a 

 small middle one m.n.co., and a small but 

 sharply defined posterior one, p.n.co. 



As the longitudinal commissures approach 

 the neuromere, they divide into distinct neu- 

 ral and haemal tracts h.tr. and nJr. In the 

 main, the fibers in each tract either terminate 

 in neuropile masses situated on the corres- 

 ponding side of the neuromere, or run straight 

 through it, the neural tracts passing above the 

 neural commissures, and the haemal tracts below the haemal commissures. 



The peripheral nerves consist of two main pairs. (Figs. 59, 60.) The 

 anterior pair is the more complex. Owing to the central relation of its fibers, 

 its position on leaving the cord, and its relation to the great muscle masses, it may 

 be spoken of as a haemal nerve. It is comparable with the motor or ventral root 

 of a vertebrate spinal nerve. It is a mixed, non-ganglionated nerve, and contains 

 general cutaneous, somatic motor, cardiac, and visceral, or intestinal elements. 



Fig. 59. — The branchial and abdominal 

 neuromeres of a young Limulus three inches 

 long. The intestinal plexus, i. pi, longitudinal 

 abdominal, /. ab, general cutaneous, g. cut, seg- 

 mental cardiacs, c, and the great branchio- 

 thoracic nerves, b. th, are shown. 



