APPENDIX B 



651 



Class 3. The Arachnids and Allies (Chelicerata). No antennae; body consisting 

 of a cephalothorax and abdomen, the former furnished with six pairs of append- 

 ages — pincerlike chelicerae and pedipalps, and four pairs of legs. Respiration by 

 book lungs or tracheae in terrestrial forms, by book gills in aquatic ones. This 

 varied class includes the marine euiypterids ( *Eurypterida, Figs. 25.3 and 27.6) 

 and king crabs (Xiphosurida, Limulus), the sea spiders (Pycnogonida), the water 

 bears (Tardigrada), and the parasitic tongue worms (Pentastomida), in addition 



Fig. B.20. Three poisonous arthropods, the bite or sting of which is painful but not dan- 

 gerous to man. Left, Scolopendra, a large centipede common in the southern United States. 

 Upper right, Centruroid.es gracilis, the slender scorpion. Lower right, Pachylomerus audouini, 

 a southern trap-door spider, with its silk-lined, hinge-lidded tube nest removed from the 

 ground. {Courtesy Ward's Natural Science Establishment, Inc., except scorpion, U.S. Depart- 

 ment of Entomology and Plant Quarantine.) 



to the great group Arachnida, the subclass which includes most living chelicerates. 

 These true arachnids include the scorpions, the whip scorpions, about 20,000 

 species of spiders, the harvestmen or "daddy longlegs," and the mites and ticks, 

 in addition to other less well known types of organisms. 



Class 4. The Millipeds and Allies (Diplopoda, Pauropoda, Symphyla). The 

 diplopods are the millipedes, or thousand-legged worms. They have long sub- 

 cylindrical bodies with from 25 to more than 100 segments, most segments bearing 

 two pairs of appendages; the head has one pair of antennae and a pair of mandibles 

 and of maxillae. The sex openings are anterior. The pauropods and symphylans 



