234 INVERTEBRATE ZOOLOGY 



1. Order Xiphosura. — Cepthalothorax large; abdominal somites 

 fused, terniiuating in a long spine. Limulus. 



2. Order Eurypterida. — Extinct; cephalothorax small; abdomen 

 twelve jointed; Eurypterus, Fterygotus. 



b. Subclass Arachnida. — Chiefly air breathers; usually four pairs 

 walking legs, one pair pedipalps, and one pair chelicerae on 

 cephalothorax. 



1. Order Scorpionida. — Abdomen segmented, posterior region 

 slender, flexible, frequently ending in sting; pedipalps and cheli- 

 cerae chelate. Buthus, Uropleclus, Pandinus, Centrums. 



2. Order Araneina. — Deep constriction between cephalothorax 

 and abdomen; abdomen unsegmented; spinning organs near caudal 

 extremity. Epeira, Agalena, Eruypelma, Lycosa. 



3. Order Acarina. — Broad, unsegmented abdomen, not constricted 

 at union with cephalothorax; frequently with piercing beak. 

 Boophihis, Sarcoptes, Dermacentor, Hydrachna. 



4. Order Pseudoscorpionida. — Resembling ^scorpions but abdomen 

 not divided into two regions; no sting; chelicerae spinning organs. 

 Chelifer. 



5. Order Solpugida. — Extremely large chelicerae on head. Eniero- 

 bates, Galeodes, Dalames. 



6. Order Phalangida. — Four pairs exceedingly long legs. Liobo- 

 num, Phalangium. 



7. Order Linguatulida. — Wormlike; two pairs degenerate legs of 

 which only claws remain. Pentastoma, Linguatula, Arviillifer, 

 Porocephahis. 



III. Class Pycnogonida. — Marine; abdomen vestigial; cephalothorax 

 bearing terminal suctorial proboscis and seven pairs very long legs. 

 Nymphon, Pycnogonum. 



IV. Class Tardigrada. — Microscopic; aquatic; four pairs unsegmented 

 appendages, each with terminal claws. Marcobiotus. 



^V. Class Onycophora. — Wormlike; numerous, short, paired legs, ringed 

 but not jointed; tracheae. Peripatus. 

 VI. Class Myrientomata. — Minute; antennae and cerci lacking. 



1. Order Protura. — Three pairs legs on thorax; three pairs vestigial 

 legs on abdomen. Acerentomon. 

 ^ VII. Class Diplopoda. — Terrestrial; body cylindrical; numerous seg- 

 ments, mostly bearing two pairs of short legs inserted pear midline. 

 Julus, Glomcris, Spirobolus, Polyxenus, Parajidus. 



VIII. Class Chilopoda. — Terrestrial; tracheate; numerous segments; 

 body flattened; single pair legs to a somite, lateroventral. Lithobius, 

 Geophilus, Scutigcra. 



IX. Class Symphyla. — Eleven or twelve pair legs. Scolopendrella. 



X. Class Pauropoda. — Minute; nine pairs legs. Pauropus. 



References 



(See general references at close of Chapter I) 

 Banks, N. 1904. A Treatise on the Acarina, or Mites. Proc. U. S. 

 Nat. Mus., 28: 1-114. 



