520 



COMPARATIVE ANATOMY 



CHAP. 



pierced by the oesophagus ; the greater part of this mass, which lies 

 behind the oesophagus, represents the fused ventral chord, from which 

 the nerves radiate. 



The disappearance of a segmented abdominal ganglionic chain in the Arach- 

 noidea may have been brought about by various co-operating factors : (1) by a forward 

 displacement of the ganglionic masses, and the fusing of the same with the thoracic 

 mass ; (2) by the fusing of several abdominal ganglia to form one or two abdominal 

 masses ; (3) perhaps also by the running of the ganglia into the nerve trunks which, 

 paired or externally unpaired, run through the abdomen ; and (4) by the concentra- 

 tion of the whole ventral chord into one single thoracic ganglionic mass. 



In the Scorpionidce, where the nervous system in the abdomen is still most richly 

 segmented, there are 7 abdominal ganglionic masses, 3 in the pre-abdomen, 3 in the 



FIG. 365 Central nervous system (thor- 

 acic ganglionic mass) of Gamasus, diagram- 

 matic (after Winkler). g, Portion lying over 

 the oesophagus (o)= brain ; ug, portion lying 

 under the oesophagus (infra-cesophageal gang- 

 lionic mass) ; 1-6, nerves of the 6 pairs of limbs ; 

 1, of the chelicerse (mandibles) ; 2, of the pedi- 

 palps (maxillse) ; nz, nerve of the tongue ; ns, 

 visceral nerve ; tn, nerve of the maxillar palp. 



FIG. 366. Nervous system of Pentas- 

 tomum taenioides (after R. Leuckart). o, 



oesophagus ; m, anterior portion of the chylific 

 stomach ; on, oesophageal nerves ; os, oeso- 

 phageal ring running over the oesophagus ; 

 ug, infra-oesophageal ganglionic mass. 



post-abdomen, and 1 on the boundary between the two, which as yet cannot be 

 certainly assigned to the one or the other. 



The longitudinal commissures of the Arachnoidean ventral chord are, almost 

 everywhere where they are distinguishable, fused in the middle line into an exter- 

 nally single median longitudinal strand. 



The nervous system of the endo-parasitic Linguatulidce (Fig. 366) is much reduced. 

 It is restricted to one ganglionic mass lying beneath the oesophagus and an apparently 

 double commissure embracing the oesophagus, in which no special cerebral swelling 

 can be made out. This great reduction of the brain is chiefly due to the degeneration 

 of the eyes. 



A sympathetic nervous system has been proved to exist in various Arachnoidea 

 (Scorpionidce, Araneidce, Acaridce), and consists of an unpaired nerve connected with 

 the brain by paired nerves and running along the oesophagus and stomach. Ganglia 

 connected with the ventral chord have also been described as belonging to the 

 sympathetic nervous system. 



