\i PHYLUM ARTHROPODA 061 



first pair of legs terminate in a many-jointed flagellum. The 

 organs of respiration are two pairs of book-lungs on the second 

 and third segments of the abdomen. 



This order includes the Scorpion-spiders (Fig. 541). 



Order 4. — Solpugida. 



Arachnida with three regions — head, thorax (of three segments) 

 and abdomen (of ten segments). The chelicerae are chelate ; the 

 pedipalpi elongated and leg-like. The organs of respiration are 

 tracheae. 



This order includes Galeodes (Fig. 542). 



Order 5. — Phalangida. 



Arachnida with an unsegmented cephalothorax, and an abdomen 

 of six segments. The chelicerae are chelate, the pedipalpi leg-like. 

 The organs of respiration are trachea?. No spinning glands are 

 developed. 



This order includes the Harvest-men. 



Order 6. — Araneida. 



Arachnida in • which the body is composed of an undivided 

 cephalothorax and an unsegmented abdomen, which is usually soft 

 and rounded, and attached to the cephalothorax by a narrow neck. 

 The chelicerse are sub-chelate, with poison-glands ; the pedipalpi 

 simple. The organs of respiration are book-lungs alone, or book- 

 lungs combined with tracheae. 



This order comprises all the true SpidersjCEig. 543). 



Order 7. — Acarida. 



Arachnida in which the body exhibits no division into regions. 

 The mouth-parts are adapted either for biting or piercing and 

 sucking. The organs of respiration, when present, are in the 

 form of tracheae. 



This order includes the Mites and Ticks (Figs. 546 and 547). 



Order 8. — Xiphosura. 



Arachnida in which the body consists of a cephalothorax 

 covered over by a broad carapace, and an abdomen of seven 

 firmly united segments, with a long narrow tail-piece or telson. 

 The cephalothorax bears a pair of short chelate appendages 

 and five pairs of legs. The abdomen bears in front a pair of 

 . united plate-like appendages, forming the operculum, followed 

 by five pairs of flat appendages overlapped by the operculum. 

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