ANTENNATATHE NERVOUS SYSTEM 



465 



generally, not always, proceeds in both from 

 behind forward. A junction of the fused ganglia 

 of thorax and abdomen to form a large thoracic 

 ganglionic mass may even take place (as in the 

 Bmchyura and many Copepoda) ; such cases occur 

 in the Diptera and Rhijnclwta. Although the 

 larva? generally possess a less concentrated 

 nervous system than the imagines, so that the 

 progressive concentration can often be followed 

 ontogenetically in the same species, this is not 

 always the case, in fact the very reverse occasion- 

 ally occurs. The interesting relation between 

 the nervous system of the larva and that of the 

 imago will be again referred to. 



From the ganglia of the ventral chord of the 

 trunk (thorax and abdomen) arise the nerves for 

 its integument, musculature, glands, and limbs. 

 The 2 ganglia of a double ganglion are always 

 closely contiguous, and appear as one mass con- 

 sisting of two halves ; the longitudinal com- 

 missures, however, which unite the consecutive 

 ganglia very often remain separate. A sym- 

 pathetic nervous system seems present in all 

 Antenriata. 



Myriapoda (Fig. 323). 



One ganglion is found in each trunk segment. The 

 ganglia are mostly united by distinctly separate longi- 

 tudinal commissures. In the Pauropoda and Symphyla, 

 however, the ventral chord is a median strand with con- 

 secutive swellings, corresponding with the ganglia, and in 

 this strand the longitudinal commissures are not separate. 



The 2 anterior trunk ganglia (or in Symphyla only 

 the first) generally form with the sub-oesophageal ganglion 

 a single mass, in which, however, the original composition 

 can easily be made out. The limbless anal segment has 

 no separate ganglion, and the ganglia of the 2 or 3 pre- 

 ceding segments are fused together. 



The double segments of the Diplopoda each have 2 

 ganglia. 



Hexapoda. 



Apterygota. In this division we have very good 

 illustrations of the concentrated and non - concentrated 

 nervous systems. The Thysanura have a non-concentrated 



FIG. 324. Central nervous system of Machilis maritima 

 (after Oudemans). au, Eye ; lo, lobus opticus ; g, brain ; an, 

 antennal nerve ; oe, oesophagus passing between the cesophageal 

 commissures ; usg, infra - cesophageal ganglion ; I III, thoracic 

 ganglia ; 1-8, abdominal ganglia, the last (8 a b c) consisting of three 

 fused ganglia ; s, sympathetic nervous system of the ventral chord. 



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