256 



COMPAEATIVE ANATOMY. 



Among the Araclinida the ventral ganglia are very often reduced 

 and fused. They are all characterised by the close connection 

 between the cerebral ganglia, and the venti'al chord, owing to the 

 extreme shortness of the commissures. 



The nervous system is most richly segmented in the Scorpions. 



The feebly developed cephalic ganglion 

 gives off two short commissures to the 

 ventral chain, which consists of 8 ganglia. 

 The first of these is remarkable for its 

 size, and appears to be homologous with 

 the single large ganglion in the cepha- 

 lothorax of the Spiders. As in them it 

 is the point of origin of the pedal nerves, 

 and must therefore be considered as com- 

 posed of several ganglia. The three suc- 

 ceeding ganglia are also placed in the 

 cephalothorax, and the four last, which 

 are widely separated from one another, 

 are found in the segments of the tail. 



In the Galeodea and Phrynida, as in 

 the Aranea, the ganglionic chain is re- 

 placed by a large ventral ganglion, which 

 (Fig. 130, i) is, especially in the Spiders, 

 of a radiate form, and gives off the nerves 

 for the ventral appendages, and also two 

 which run into the abdomen ; in the 

 Galeodea branched nerve-trunks are sent 

 to the segments of the abdomen. 



In all these divisions the cerebral gang- 

 lion, which is generally distinctly paired, 

 and in the Galeodea (Fig.l30,s) especially 

 large, gives off nerves for the eyes ; close 

 by the optic nerves those for the chelicerae 

 arise in the Spider, and thus we see that 

 these organs are metamorphosed antennae. 

 A complete concentration of the central portions of the nervous 

 system is seen in the Acarina, where the cerebral ganglia, which 

 are often feebly developed, may be replaced by a commissure. The 

 ventral medulla, which is large and forms a single mass, exhibits 

 numerous traces of segmentation in the arrangement of the gang- 

 lionic cells and fibrous elements ; it gives off nerves all round. 



The simple character of the nervous system in the Pycnogonida 

 is due to the decrease in number of their ganglia, owing to a reduc- 

 tion of the segments of the body ; their cerebrum is connected by 

 short commissures with the ventral medulla, which is formed of four 

 pairs of ganglia. 



Fig. 130. Nervous system of 

 Thelyplioniis caudatus. 

 s Cerebral ganglion, i Ventral 

 ganglion, o Eyes, p Palpi, 

 iji — jji^ Feet, tr Lungs, c 

 Tail-like appendage of the 

 body (after Blanchard). 



