436 ARTHROPODA 



21. The Leplostraca have twenty-one somites; they are closely related 

 to the Phyllopoda. 



22. The Thoracostraca or Podophthalmia (Schizopoda, Stomatopoda, 

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

 fused with the head to a cephalothorax. 



23 . The A rthrostraca or Edriop/i l/ni hu la have sessile eyes and have seven 

 free thoracic segments. They are divided into Amphipoda and Isopoda. 



24. The ACERATA lack antennas ; the body is divided into cephalothorax 

 and abdomen; the cephalothorax bears six pairs of appendages; the genital 

 ducts open on the seventh somite; the respiratory organs gills, lungs, 

 or trachea develop, in connection with the abdominal appendages. 



25. The Acerata are divided into Gigantostraca and Arachnida. 



26. The Gigantostraca are large, and breathe by gills. The only 

 living forms are Xiphosures. 



27. The Arachnida breathe by lungs or by tracheae derived from lungs, 

 the openings to which are on the abdomen; they have a pair of chelicerse, 

 a pair of pedipalpi, and four pairs of legs; they have in addition several 

 pairs of highly developed ocelli. 



28. The Arachnida are divided into nine orders: Scorpionida, Phry- 

 noidea, Microthelyphonida, Solpugida, Pseudoscorpii: Phalangida, 

 Araneina, Acarina, and Linguatulida. 



29. The Scorpionida have chelate pedipalpi and a postabdomen 

 terminated by a sting. 



30. The Phrynoidea have the first pair of legs tactile and not used in 

 walking, and a continuous cephalothorax. 



31. The Microthelyphonida and the Solpugida have three 'thoracic' 

 segments free. The Microthelyphonida have a long, jointed postab- 

 domen, lacking in the Solpugida. 



32. The Pseudoscorpil resemble the Scorpionida, but lack the post- 

 abdomen and sting. 



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



34. The Araneina have an unsegmented abdomen, bearing four or 

 six spinnerets and numerous spinning glands. They &re divided into 

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

 two trachea-. 



35. The Acarina have cephalothorax and abdomen fused and the 

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



36. The LinguafiiUda are complete parasites, ribbon-like and with- 

 out legs; the young live in the lungs and liver. 



37. The Tardigrada and Pycnogonida agree with the Arachnida in the 

 number of walking legs. Their position is very uncertain. 



