Introduction to Animal Morphology. 363 



CHAPTER XLIV. 



DIVISION 2. — TRACHEOPNOA {Schmardd). 



Air-breathing Arthropods with one pair of antenna, 

 and either a cavity for the reception of air, or else a 

 system of tubes (tracheae) conveying it through the 

 body. Three classes are included : — , 



Class 2. Arachnoidea ( V. der Hocvcn) — mostly ter- 

 restrial, with the head and thorax fused into a cepha- 

 lothorax, bearing four pair of limbs. The abdomen 

 is limbless, rarely with a post-abdomen, and the eyes 

 are simple. The mandibles (as such) are absent, but 

 their palps become developed in Scorpions as chelcBy 

 or food-graspers, and the antennae act as mandibles. 

 The maxillary palps become the first and second am- 

 bulatory legs, while the third and fourth pair are the 

 pro- and meso-thoracic limbs. Like Crustacea, they 

 often moult, and at that time lost parts are restored. 

 They do not part with their limbs as readily as Crus- 

 taceans, which, under circumstances of seizure or 

 fright, often voluntarily sever one of their legs between 

 the basi- and coxopodite. 



The nervous system in the adult is concentrated, 

 with short commissures, or may be simplified by the 

 abortion of segments, as in Pycnogonidae. In Scor- 

 pions the epipharyngeal ganglion is laterally con- 

 stricted, supplying the stemmata, and sending two 

 commissures to the limb-supplying hypopharyngeal 

 ganglion, which consists of seven fused ganglia, three 

 post-oral cephalic, three thoracic, and one abdominal : 

 behind it are three abdominal and four post-abdomi- 



