COMPAEATIVE MOEPHOLOGY OF THE GALEODID.^. 341 



VI. The Nervous System. 



The great condensation of the anterior end of the body of Arachnids, in the interests 

 of locomotion, and in contrast to the sac-like specialization of the abdomen as a receptacle 

 for food and the genital products, is reflected in the nervous system. In Galeodes, 

 in spite of the fact that the cephalothorax in this animal has retained three free 

 segments, the central nervous system is a compact mass composed of a number of fused 

 ganglion-centres grouped round the oesophagus. 



The Central Ganglionic Ilass. — The dorsal or brain portion of this mass consists of 

 several ganglionic centres, of Mhich the most important are those from which the optic 

 nerves arise (PI. XXXI. fig. 2). Behind these there appear to be the centres of the 

 nerves innervating the mid-gut (see, however, below^, w'hile in front there is a nerve- 

 centre in the middle line above the oesophagus, Avhich belongs to the labral nerves 

 (PI. XXXI. fig. 6, L). On each side of this, and also slightly above the oesophagus, are 

 the centres of the cheliceral nerves (ch). 



The ventral or trunk portion of the nerve-mass consists of a far greater number of 

 ganglionic centres. The first and largest are those from which the pedipalpar nerves (pp) 

 arise, situated just below the cheliceral centres and slightly at the sides of the oesophagus. 

 The pedipalpar nerve-centres are followed along each side of the nerve-mass by a row of 

 four nerve-centres supjilying the four pairs of legs (PI. XXXI. fig. 2, 1.2,3.4). Between 

 these pedipalpar and leg nerve-centres, and immediately below the oesophagus, there 

 appear to be several median nerve-centres ; these are very large and important, 

 anteriorly between the pedipalpar centres, and posteriorly between those of the last 

 pair of legs. These median centres are in direct connection with the nerves running 

 into the abdomen. 



Not only is the condensation of the anterior end of the body thus reflected in the 

 fusion of the ganglionic centres, but the arrangement of these latter appears to reflect 

 the changes which have taken jilace in the arrangement of the anterior pairs of limbs. 

 As we have already seen, the Arachnids are Arthropods specialized by the shifting 

 forward of the first two pairs of limbs, the tilting up of the cbelicerse to a position above 

 the mouth and prostomivim, and to a less extent of the pedipalps, to a position at the 

 sides of the latter. This forward and upw^ard movement is clearly recorded in sections 

 of the brain. The cheliceral nerve-centres, as is well known *, belong primarily to the 

 infra-oesophageal system, and have secondarily moved up the oesophageal commissure to 

 positions more or less supra-oesophageal. In Galeodes the cheliceral ganglia have 

 travelled far up along the oesophageal commissures, so as to be practically supra- 

 oesophageal. In this process they seem to Jiave pushed the optic ganglia backward ; at 

 least, this action is suggested by an examination of sagittal sections such as that shown 

 in PI. XXXI. fig. 2. 



The variations in the forms of the central ganglionic mass in Arachnids reflect the various 

 specializations of the cephalothorax, and show, as we should expect, many differences in the relative 

 positions of the ganglia. 



* Cf. Mctschnikoff (54) and Balfour (i). 



