GENERAL ORGANOLOGY 



113 



of nervous elements. In the medusae such a place is the rim of the bell ; 

 consequently a stronger nerve-cord much richer in ganglion cells is found 

 here. This, as well as the nerve-ring and the live radial nerves of 

 echinoderms, may be called a central system, thereby distinguishing the 

 rest of the nervous network as the peripheral nervous system. 



Ganglionic Central Nervous System. Numerous transitional 

 forms lead to the ganglionic central nervous system of the worms, molluscs, 

 and arthropods (fig. 77). The central nervous 

 system here consists of two or more ganglia; each 

 ganglion being a bunch of regularly arranged 

 nerve-fibres and ganglion-cells. The former con- 

 stitute the centre of the mass, and, since they 

 cross in all directions giving off branching den- 

 drites, they appear like fine granulations. The 

 ganglion-cells, on the other hand, collect in a 

 thick layer around the granules. The peripheral 

 nerves, and also the commissures, the cords con- 

 necting similar ganglionic masses, extend out- 

 wards from the ganglia. 



Supraoesophageal (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 occurrence are two ganglia, 

 which lie dorsally above the pharynx, and hence 

 are called the supra-ccsophageal or cerebral ganglia. 

 If other ganglia occur, they lie ven trail v ar.d 

 below the digestive tract (ventral nerve-cord}. 



In the Ladder Nervous System of annelids 

 and arthropods (fig. 78), numerous pairs of 

 ganglia (in the example before us, nine) lie in 

 serial order on the ventral side of the animal, and 

 are connected by longitudinal commissures (con- 

 nectives'), and also by transverse commissures 

 connecting the left and right ganglia. The first 

 pair of the series is the infra-oesophageal ganglion, 



which sends out connectives right and left, surrounding the pharynx, 

 to the supraoesophageal ganglion. The supra- and infra-cesophagral 

 ganglia together with the cesophageal connectives form the cesophageal 

 ring, a nerve-ring surrounding the oesophagus. 

 8 



FIG. 78. Ladder 

 nervous system of sowbug 

 (Porcellio scaber) (after 

 Leydig). A, brain: B, 

 ventral cord, connected 

 with the brain by the 

 cesophageal commissures 

 b, a cord formerly re- 

 garded as syrapatheticus. 



