F. ARTHROPODA, SUMMARY OF IMPORTANT FACTS. 499 



appendages; the genital ducts open on the seventh somite; the 

 respiratory organs gills, lungs, or trachea develop from 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 chelicene, 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, 

 Phrynoidea, Microthelyphonida, Solpugida, Pseudoscorpii : Pha- 

 langida, 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 postabdomen, lacking in the Solpugida. 



32. The Pseudoscorpii resemble the Scorpionida, but lack the 

 postabdomen 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 are divided 

 into Tetrapneumones, with four lungs, and Dipneumones, with 

 two lungs and two tracheae. 



35. The Acarina have cephalothorax and abdomen fused and 

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



36. The Linguatulida are complete parasites, ribbon-like and 

 without 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. 



38. The MALA co POD A are intermediate between Annelida and 

 Insecta. They have indistinctly segmented bodies with parapodia- 

 like feet, segmental organs, and tracheae. 



39. The INSECTA breathe by tracheae; the head bears four pairs 

 of appendages: antennae, mandibles, maxillae, labium; since 

 tracheae are present the circulatory system is reduced; the repro- 

 ductive organs open at the hind end of the body. 



40. The Insecta are divided into Chilopoda and Hexapoda. 



41. The Chilopoda have numerous body segments with a pair 



