PHYSIOLOGY OF THE INVERTEBRATA. 323 



cells are confined to the ganglia ; in this respect the nerves 

 of the leech are like those of Astacus and the spinal cord of 

 the Vertchrata. 



" The nervous system of the PolycJwta usually consists of 

 a chain of ganglia — one pair for each somite — connected 

 together by longitudinal and transverse commissures, which 

 diverge between the cerebral ganglia and the succeeding 

 pair, to allow of the passage of the ojsophagus. The most 

 important differences presented by the nervous systems of 

 the Fohjclicvta result from the varying length of the transverse 

 commissures. In Vcrmilia, Ser}nda, Sahella, these commis- 

 sures are very long, so that two distinct and distant series 

 of ganglia appear to run through the body, while, in Ncptliys, 

 the two series of ganglia are fused into a single cord 



enlarged at intervals In most Pohjchccta a very 



extensive series of visceral nerves supplies the alimentary 

 canal." 



The Nematoscolices. 



In the Nematoidea the nervous system consists of a nerve- 

 ring surrounding the oesophagus. From this ring proceed 

 six nerves in an anterior, and two in a posterior direction. 

 Two of the anterior nerves proceed in the lateral lines — that 

 is, one in each — and four in the interspaces between the lateral 

 and median lines. The posterior nerves proceed to the tip 

 of the tail — one in the dorsal, and the other in the ventral 

 median line of the body. Near to the nerve-ring, in front 

 and behind it, arranged in dorsal, ventral, and lateral groups, 

 lie certain ganglia. These are respectively known as dorsal 

 or supra-oesophageal, ventral or sub-cesophageal, and lateral 

 ganglia. In addition to these, there are groups of ganglia 

 in the median and lateral lines, in the posterior part of the 

 body ; these are known as caudal ganglia. 



In the Acanthoccphala, represented by Echinorhyncus, the 

 nervous system consists of a simple ganglion, which is situated 



