XI 



PHYLUM ARTHROPODA 



653 



(Fig. 563), or two pulmonary sacs and a system of tracheae (Fig. 564). 

 Tracheae are present in the Phalangida and also in the majority 

 of the Acarida. 

 In the Xipho- 

 sura the organs 

 of respiration 

 are external 

 appendages or 

 gills (book-gills), 

 in the shape of 

 delicate laminae 

 attached to the 

 abdominal ap- 

 pendages (Fig. 

 565). 



The nervous 



* 

 system is, in 



most instances, 

 more concen- 

 trated than in 

 the Scorpions. 

 There may be 

 one or two 

 separate abdo- 

 minal ganglia 

 behind the mass 

 formed by the 

 united cephalo- 

 thoracic and 

 anterior abdo- 

 minal (Pseudo- 

 scorpionida, 

 Pedipalpida, 

 some Araneida, 

 Solpugida, Pha- 

 langida). In 

 most of the 

 Araneida and 

 in the Acarida 

 all the abdo- 

 minal are united 

 with all the 

 cephalothoracic 

 ganglia to form 



a single mass perforated by the oesophagus, the part lying behind, 

 which is much the larger, representing the ventral nerve-cord. 

 Sense-organs. Eyes are present in all except in some of the 



