322 



PHYSIOLOGY OF THE INVERTEBRATA. 



ganglia are connected by two nerves, which embrace the 

 pharynx, with the sub-cesophageal ganglia. The latter ganglia 

 are the first of the ventral ganglionated nerve-cord. This 

 ventral nerve has a double ganglionic enlargement in every 

 segment posterior to the third. A large nerve, which divides 

 and sub-divides, proceeds forward from each of the cerebral 

 ganglia. 



Pour or five nerves run backward from the upper part of 

 each half of the circum-pharyngeal ring, and are distributed 

 in the muscular walls of the pharynx. Nerves are also given 

 off from the lower portion of this ring to the muscles of the 

 fourth segment. Two pairs of nerves from each bilateral 

 ganglionic enlargement of the ventral cord, proceed to the 

 viscera and muscles of each segment. Two nerves, one from 

 each side, pass off from the ventral nerve at a point nearly 

 half-way between the double ganglionic masses. These 

 supply the posterior sides of the mesenteric septa. 



When examined under high power the nerve-rods of 



ABC 

 Fig. 62. — Nervous Systems of Polych^;ta. 



A = Polynbe squamata. 



B = SabelUi Jlahellata. 



C = Nereis 



a = cerebral ganglia. b = oesophageal commissures. 

 c = longitudinal commissures of ventral ganglia, 



Lumhricus are seen to contain a large number of nerve- 

 cells along with the nerve-fibres. This is a characteristic 

 feature of Lumhricus and Peripatus. In Hirudo the nerve- 



