436 ARTHROrODA 



21. The Lcplostracj have twciity-one somites: they arc closely related 

 to the riiyllopoda. 



22. The Tlioraroslraca or rojoplilluiliiiia (Schizopoda, Stomatopoda, 

 Decapoda) have stalked eyes and some or all of the thoracic somites 

 fused with tlie head to a cephalothorax. 



23 . The - 1 rth rostra Id or l\drioplithahnia have sessile eyes aird have seven 

 free thoracic segments. They are di\ided into Aniplriiunia and Isopoda. 



24. The AcEK_\TA lack antennx ; the 1 loih- is di\"ided into cephalothorax 

 and abdomen; tdie cephalothorax hears six pairs of a]ij)endaL;es; the genital 

 ducts open on the se\-enth somite; the respiratory organs — gills, lungs, 

 or trachea — develop, in connection with the ahdominal a]>[K>ndages. 



25. The Acerata are ilixided into Gigantostraca and Arachnida. 



20. The Giganiostrjca are large, and breathe by gills. T]\e only 

 living forms are Xiphosures. 



27. The Arac/iiiiJj lireathe by lungs or by trachea- derived from lungs, 

 the openings to which are on the abdomen; thev ha\"e a pair of chelicera', 

 a pair of pedipalpi, and four pairs of legs; they ha\'e in addition se\'eral 

 pairs of highlv developed ocelli. 



2S. The Arachnida are di\"ided into nine orders; Scorpionida, Thry- 

 noidea, ^Nlicrothelyphonida, Solpugida, Tseudoscorpii: Phalangida, 

 Araneina, Acarina, and Tinguatulida. 



20. The ScorpioinJa have chelate pedipalpi and a postabdomen 

 terminated ]>y a sting. 



30. The riirynoidca have the hrst pair of legs tactile and not used in 

 walking, and a continuous cephalothorax. 



31. The MicrolliclyplioiiiJa and the SoIpii;:,ichi h;xve three 'thoracic' 

 segments free. The ^Nlicrothelyphonida ha\e a long, I'ointed postab- 

 domen, lacking in the Solpugida. 



32. The rsciiJoscorpii resemlile the Scorpionida, Init lack the post- 

 abdomen and sting. 



33. The Plialjiigida have very long legs and spider-like bodies. 



34. The Araiiciiiu ha\-e an unsegmented alxlomen, bearing four or 

 six spinnerets and numerous spinning glands. They are divided into 

 Tetrapneumones, with four lungs, and Dipneumones, with two lungs and 

 two trachea-. 



35. The Acariini have cephalothorax and abdomen fused and the 

 mouth parts for sucking. Several species are parasitic on nnin. 



36. The Liiioua'ulida are complete parasites, ribbon-like aird with- 

 out legs; the young li\-e in the lungs and liver. 



37. The Tardigrada ami Pyoiogoiiida agree with the Arachnida in the 

 number of walking legs. Their position is \erv uncertain. 



