1 PHYLUM ARTHROPODA 645 



A pair of comb-like pectines occur on the second segment of the 

 pre-abdomen. The organs of respiration are four pairs of book- 

 lungs in the third, fourth, fifth and sixth segments of the pre- 

 abdomen. 

 This order includes the Scorpions. 



ORDER 2. PSEUDOSCORPIONIDA. 



Arachnida in which there is a continuous cephalothorax, some- 

 times marked dorsally with two transverse grooves, and a broad 

 abdomen, not divided into pre- and post-abdomen, and not pro- 

 vided with a sting. The chelicerae are very small, the pedipalpi 

 similar to those of the Scorpions. The organs of respiration are 

 a system of tracheae. A pair of spinning glands are present. 



This order includes the Book-scorpions (Fig. 550). 



ORDER 3. PEDIPALPIDA. 



Arachnida in which the body consists of unsegmented cephalo- 

 thorax and flattened abdomen of eleven to twelve segments. The 

 chelicerse are simple, the pedipalpi simple or chelate, and the 

 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. 551). 



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. 552). 



ORDER 5. PHALANGIDA. 



Arachnida with an unsegmented cephalothorax, and an abdomen 

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

 The organs of respiration are tracheae. No spinning glands are 

 developed. 



This order includes the Harvestmen. 



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 chelicerae 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 Spiders (Fig. 553). 



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