GENERAL OROANOLOOT. 



123 



rounded bunch of regularly arranged nerve-fibres and ganglion- 

 cells. The former constitute the centre of the mass, and, since 

 they cross in all directions, give the appearance of fine granula- 

 tions; this fact has led to the unsuitable, because misleading, name 

 of < Leydig's dotted substance/ The ganglion-cells, on the other 



FIG. 74. Third abdominal ganglion of a crayfish. (After Retzius.) C, connective or 

 longitudinal commissure; G, ganglion cell layer; g' ganglion cell whose neurites 

 enter the connective; 2 , ganglion cell whose neurites enter the peripheral nerve ; 

 _L, Leydig's dotted substance; JV, peripheral nerve. 



hand, collect in a thick layer around the dotted substance. The 

 peripheral nerves, and also the commissures, the cords connecting 

 similar ganglionic masses, extend outwards from the ganglia. 



Supracesophageal (or Cerebral) Ganglia. Since most animals 

 are symmetrical, the ganglia occur in pairs; left and right ganglia 

 correspond to one another and are connected simply by a cord of 

 nerve-fibres, the transverse commissure. Of most constant occur- 

 rence are two ganglia, which lie dorsally above the pharynx, and 

 hence are called the supracesophageal or cerebral ganglia. If other 

 ganglia occur, they lie ventrally and below the digestive tract 

 (ventral nerve-cord). 



