THE BRAIN. 43 



sented by the supra-stomodaeal commissure with its anterior median, and two 

 lateral, ganglia. These nerves and ganglia are without doubt very ancient 

 structures, and their position and mode of development clearly indicate that they 

 belong to a different system of nerves from those in the remaining part of the head 

 or trunk. 



There is probably a distinct post-cesophageal ganglion and commissure 

 belonging to this system, although I have not succeeded in locating it, or in dis- 

 tinguishing it from the more anterior post-oral commissures. The supra-stomo- 

 daeal commissure always sends nerves to the labrum, or rostrum, which receives 

 nerves from this source only. The innervation of this pair of appendages, their 

 median position in front of the mouth, and between the right and left halves of 

 the forebrain, distinguish them from all others, and indicate their probable origin 

 from tentacle-like organs of some very remote ancestor. 



Originally the stomodaeal neives appear to have been intimately connected 

 with the two median longitudinal nerves of the trunk, i.e., with the median cardiac, 

 on the haemal side, and the median sympathetic on the neural. Both these con- 

 nections are lost in the adults of the higher arachnids, i.e., in Limulus, although 

 in the scorpion the connection with the cardiacs seems to be retained. 



The dividing line between the ccelenterate nervous system of the primitive 

 head and that belonging to the bilateral outgrowth from it, cannot be accurately 

 determined, and indeed there is no reason to suppose the two were ever distinct 

 systems, the post-oral nerves being merely extensions of the older ones in 

 the head. 



III. THE FRAME-WORK OF THE NERVOUS SYSTEM. 



The nervous system of segmented animals may be reduced to a system of 

 longitudinal and transverse strands or cords. 



Longitudinal Cords. In the arachnids, eight longitudinal nerve cords may 

 be recognized: a median haemal one, from which arises the cardiac ganglion; a 

 median neural one, or middle-cord (Mittelstrang of Hatschek), from which arises 

 the so-called median sympathetic nerve; a pair of ventral cords, which give rise 

 to the main axial nervous system, or neuron (brain and spinal cord) ; the paired 

 pericardials; and the lateral sympathetics. 



The Median Nerve, " Median Sympathetic," or "Middle-Cord," of arthropods 

 appears to have extended backward, from the posterior part of the oesophageal 

 region, or of the circumoral nerve ring, the whole length of the body. I have not 

 been able to determine the peripheral distribution of its fibers. The main nerve 

 and its sheath undergo many modifications. In the higher arthropods and verte- 

 brates, the nerve itself atrophies and ceases to form a functional part of the nervous 

 system. It serves, however, as a center for the development of voluminous, 

 resistent envelopes from which is evolved the notochord. The history of the middle 

 chord will therefore be described in the chapter on the evolution of the notochord 



