NERVOUS SYSTEM OF ARTHROPODA. 



259 



wliicli arise from the last ganglion. A similar difference in the 

 number of the ganglia of the ventral chord obtains in the Diptera, 

 where the most primitive characters are seen in Pulex : 3 thoracic 

 and 7-8 abdominal ganglia. In others there is generally a consider- 

 able reduction by the fusion of the thoracic, or of the abdominal 

 ganglia, or of both (Fig. 131, C). With this is connected the com- 

 plete fusion of the ventral chord into one somewhat long knot, in 

 the parasitic Pnpipara. We find the same characters in the Strep - 

 siptera. As to the Lepidoptera there is more uniformity in them, 

 a constant number of ganglia being found 

 in the larva, while when it is metamor- 

 phosed into the butterfly the mode of fusion 

 appears to be essentially the same in all. 



§ 200. 



The visceral nervous system of the 

 Arthropoda shows signs of some common 

 characters, together with great variation in 

 particular points. Among the Crustacea 

 nerve-filaments pass from the oesophageal 

 commissure to the enteron, or a nerve passes 

 to the enteric canal from the ventral chord. 

 (In Astacus from the last ganglion also.) 



Even in the Arachnida nerves are given 

 off partly from the cerebrum, and partly 

 from the ventral ganglion to the enteron ; 

 in the Opilionea the posterior ones are pro- 

 vided with a large number of ganglia. 



In the Insecta and Myriapoda the break- 

 ing up of the visceral nervous system into 

 several portions has been more generally 

 made out; we will therefore examine the 

 arrangement of it more closely. One part 

 forms the so-called paired system, which 

 consists of two branches running back- 

 wards from the cerebral ganglion to the 

 sides of the oesophagus ; these form a simple 

 chain of ganglia (Fig. 132, s' s") on either 

 these ganglia varies, 

 plexus-like connection with the unpaired system, to determine to 

 which system they belong. The unpaired system (r r 1 ) arises in a 

 ganglion which lies in front of the cerebrum, and is more or less 

 connected with it. From this ganglion a thicker nerve (r) passes 

 backwards over the oesophagus to the stomach, and forms a 

 plexus with the branches of the paired system ; from this plexus 

 the neighbouring parts, especially those of the digestive system, are 

 innervated. In many Insects this nerve (N. recurrens) forms a single 

 ganglion (Coleoptera and Orthoptera), in others several (Lepidoptera) 



s 2 



Fig. 132. Supra-cesophageal 

 ganglion and visceral ner- 

 vous system of one of the 

 Lepidoptera (Bonibyx 

 Mori), g s Supra-oesopha- 

 geal ganglion (Cerebrum). 

 a Antennary nerve, o Optic 

 nerve, r Azygos trunk of 

 visceral nervous system. 

 r' Its roots arising from the 

 supra-cesophageal ganglion, 

 s Paired nerve with its 

 ganglionic enlargements s's" 

 (after Brandt). 



side. The number of 

 nd it is often difficult, on account of their 



