458 INVERTEBRATE MORPHOLOGY. 



Pedipalpi are more primitive as regards the number of abdominal seg- 

 ments and their distinctness than the Aranese, yet the latter and especially 

 the Tetrapneumones show a much more primitive condition of the respira- 

 tory organs. With regard to these organs it may be stated that the con- 

 dition in which they are represented by bunches of unbranched tracheae 

 is more primitive than that in which they are branching tubes, the 

 bunched condition being probably derived by a modification of original 

 lung-books. 



TYPE ARACHNIDA. 



1 . Order Scorpionida. — Abdomen segmented and differentiated into pra&- 

 abdomen and postabdomen ; postabdomen terminating in poison- 

 spine ; pedipalps chelate ; two pairs of abdominal appendages ; 

 four of stigmata and lung-books. Suscorpius, Buthus. 



3. Order Pseudoscorpionida. — Abdomen segmented but not differentiated ; 

 no terminal spine ; pedipalps chelate ; no abdominal appen- 

 dages ; two pairs of stigmata opening into tracheae ; first pair 

 of legs adapted for locomotion. Cheli/er, Obisium, Chernes. 



3. Order SoUfugce. — Head separated from thorax with three segments ; 



abdomen segmented but undifferentiated ; no terminal spine ; 

 pedipalps palplike ; three pairs of stigmata leading into tracheae. 

 Qaleodes, Solpuga. ' 



4. Order Pedipalpi. — No distinction of head and thorax ; abdomen seg- 



mented, and either undiiterentiated or with three small segments 

 terminated by a multiarticulate fiagellum ; pedipalps leglike or 

 subchelate ; two pairs of stigmata and lung-books ; first pair of 

 legs elongated and palplike. Phrynios, Thelyphonus. 



5. Order Plialangida. — Abdomen segmented but undifferentiated and 



without appendages or terminal spine ; pedipalps leglike ; one 

 pair of stigmata leading into tracheae ; no spinning-glands. 

 Leiohunum, PJialangium, Opilio, Oonyleptus, GyphopMhalmus, 

 GHhhocellum. 



6. Order Aranex. — Abdomen unsegmented and with two or three pairs of 



rudimentary papillalike appendages bearing the openings of 

 ducts of numerous spinning-glands ; abdomen not fused with 

 cephalothorax ; pedipalps long and palplike or leglike. 



1. Suborder Tetrapneumones. — With four stigmata opening into 

 sacs containing lung-books. Mygale, Cteniza. 



3. Suborder Dipneumones. — With four or three stigmata, the anterior 

 pair opening into sacs with lung-books, the posterior one or 

 two with tracheae. Epeira, Agelena, Tegenaria, Theridium 

 Segestria, Attus, Lycosa. 



7. Order Aearina. — Abdomen unsegmented, without appendages, and 



fused with the cephalothorax ; pedipalps sometimes long and 

 leglike, sometimes chelate ; stigmata; wanting or present as a 



