. Elements of Comparative Anatomy. XEEVOUS SYSTEM OF AETHEOPODA. 259 â wliicli arise from tlie last ganglion. A similar difference in tlie number of tlie 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, G). With this is connected the com- plete fusion of the ventral chord into one somewhat long knot, in the parasitic Pu^jipara. AVe find the same characters in th
. Elements of Comparative Anatomy. XEEVOUS SYSTEM OF AETHEOPODA. 259 â wliicli arise from tlie last ganglion. A similar difference in tlie number of tlie 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, G). With this is connected the com- plete fusion of the ventral chord into one somewhat long knot, in the parasitic Pu^jipara. AVe 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 the larva, while when it is metamor- m. phosed into the butterfly the mode of fusion appears to be essentially the same in all. § 200. The visceral nervons 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 side. The number of these ganglia varies, and it is often difiicult, on account of their plexus-like connection with the unpaired system, to determine to which system they belong. The unpair
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