.PHYSIOLOGY OF THE INVERTEBRATA. 321 



central system exhibits six equatorial ganglionic masses, 

 which afterwards form a nerve-ring by the growth of bipolar 

 processes. 



The Annelida. 



The nervous system of the Gephyrea surrounds the oeso- 

 phagus, and from it a simple or ganglionated nerve-cord 

 proceeds backwards in the ventral median line. This nerve- 

 cord gives off branches. The nerve-ring surrounding the 

 cesophagus usually has a ganglionic mass. This mass is 

 connected with rudimentary eyes. 



The nervous system of the Hirudinea, and of Hirudo in 

 particular, is highly developed. It consists of large supra- 

 cesophageal ganglia, which send off five pairs of nerves to the 

 live pairs of eyes. These ganglia are connected with a sub- 

 oesophageal ganglion by a circum-oesophageal nerve-ring. 

 They also communicate with the buccal ganglia situated 

 over and in front of the mouth. From the sub-cesophageal 

 ganglion two longitudinal, ventral, and ganglionated cords 

 proceed along the median line of the ventral aspect of the 

 body. 



The ganglia of the two ventral longitudinal cords are 

 united together in pairs by transverse commissures. Each 

 pair of ganglia sends off, to the right and left, two nerves. 

 There are twenty-three pairs of ganglia on the ventral cords, 

 in addition to the sub-cesophageal ganglion, which is com- 

 posed of three or four pairs which have coalesced, and the 

 caudal ganglion, which lies in the region of the posterior 

 sucker, and is composed of seven coalesced ganglia. 



There is also a system of visceral nerves, consisting of a 

 nerve, which proceeds from the supra- oesophageal ganglia, 

 and runs above the ventral ganglionated nerve-chain, giving 

 off along its course branches to the caeca of the stomach. 



The nervous system of the Oligochccta^ as represented by 

 Lumh'icus, consists of two cerebral ganglia situated on the 

 dorsal side of the pharynx in the third segment. These 



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