166 ARTHROPOD A 



barus, Crago, Pagurus, Emerita, Callinectes, 

 Cancer, Uca) ; Isopoda (Idothea, Erichsonella) ; 

 Amphipoda (Talorchestia, Gammarus, Ca- 

 prella) . 

 Class 2. Arachnida. 



Body divided into two principal regions, ceph- 

 alothorax and abdomen; cephalothorax bears 

 sessile eyes, 4 pairs of walking legs, chelicerae 

 and pedipalpi; no antennae; respiration usu- 

 ally by tracheae or lung sacs. Some of the 

 important orders are: Scorpionida (Buthus) ; 

 Xiphosura (Limulus) ; Pseudoscorpionida 

 (Chelifer) ; Pedipalpida (Phrynus) ; Solpugida 

 (Galeodes) ; Phalangida (Phalangium) ; Ara- 

 neida (Epeira, Agalena) ; Acarina (Sarcoptes, 

 Dermacentor) . 

 Supplementary to the Arachnida. 

 Pycnogonida. (Pantopoda.) 



(Doubtfully referred to the group.) Body 

 composed of segmented cephalothorax and 

 vestigial abdomen. Legs very long, angular, 

 and containing portions of the viscera. No 

 special respiratory organs (Anoplodactylus, 

 Pallene, Phoxichilidium) . 



Class 3. Onychophora. 



Elongated bodies with some annelid-like char- 

 acters. Appendages short, numerous, and 

 creased rather than jointed. Respiration by 

 means of tracheae. (Peripatus.) 



Class 4. Myriapoda. 



Generally elongated bodies with numerous 

 jointed appendages. A distinct head bearing 

 ocelli, antennae, and jaws is present. Respir- 

 ation by means of tracheae. 



Order 1. Symphyla. 



With not more than twelve leg-bearing trunk 

 segments. A single pair of branching tracheae. 

 (Scolopendrella.) 



Order 2. Chilopoda. 



With numerous trunk segments, each with a 

 single pair of legs. First pair of trunk appen- 

 dages forming poison jaws. Body dorso- 

 ventrally compressed. (Lithobius.) 



