218 



COMPARATIVE ANATOMY 



CHAP. 



-din 



> 



Fia. 142. —Diagram 

 matic representation of 

 tliQ nervous system of 

 the Nematoda, after 

 Butsclili. 0)t, Upper, un, 

 under portion of the oeso- 

 phageal ring ; sg, lateral 

 swellings of the same ; 

 vln, medio-ventral ; din, 

 medio-dorsal longitudinal 

 nerve; c, commissures be- 

 tween the two ; hsn, pos- 

 terior lateral nerves (bur- 

 sal nerves). 



In the Acanthocephala (Fig. 172, p. 258) there lies at the base of 

 the proboscis sheath a ganglion which sends several nerves forwards 



to the proboscis sheath, the pro- 

 boscis, and the neck. Posteriorly 

 there proceed from the ganglion 

 two lateral longitudinal nerves, 

 which first enter the retinacula 

 at the posterior end of the pro- 

 boscis sheath and run in them to 

 the body wall, and then to the 

 posterior end of the body. A 

 dorso-median longitudinal nerve 

 has also been observed. In the 

 male, besides the anterior gan- 

 glion, another ganglion in the region 

 of the genital apparatus (lying 

 anteriorly on the base of the 

 withdrawn bursa) has been de- 

 scribed ; this gives off nerves to 

 the genital apparatus, and is also 

 connected by two nerves with the 

 posterior ends of the lateral longi- 

 tudinal nerves of the bodv. 



The nervous system of tlie Acantho- 

 cephala is up to the present time not 

 clearly understood. If there really is a 

 medio-dorsal longitudinal nerve it per- 

 haps corresponds with the medio-dorsal 

 nerve of the Nematoda, and then the 

 lateral longitudinal nerves perhaps repre- 

 sent the ventral longitudinal nerves, 

 which fuse in the middle line in most 

 Xcmatoda. What the relations are be- 

 tween the oesophageal ring of the Nema- 

 toda and the ganglion of the proboscis 

 sheath of the Acanthocephala, or whether 

 any such relations exist, are questions 

 which must be left on one side. 



The Gordiidce also deviate from 

 other Nematoda in the structure 

 of the nervous system. Eound 

 the rudimentary pharynx lies a 

 ganglionic mass (peripharyngeal 

 ganglion) which is much thickened, 

 chiefly ventrally, and is produced into a ventral chord (Fig. 170, p. 

 2.56). This runs backward in the middle line and swells into a caudal 

 ganglion under the terminal portion of the genital ducts. In the male 



Fig. 143.— Cen- 

 tral nervous sys- 

 tem of Hirudo 

 medlclnalis, after 

 Hermann. 



