ARTHROPODA. 153 



Order 3. Decapoda. 



Thoracic segments united with the head. Three 

 pairs of maxillipeds and five pairs of legs, of 

 which the first bear heavy pincers. (Homarus, 

 Cambarus, Crago, Pagurus, Emerita, CalH- 

 nectes, Cancer, Uca.) 



Order 4. Stomatopoda. ■; 



Five anterior thoracic legs are maxillipeds. 

 Eyes stalked. Gills borne on abdominal seg- 

 ments. (Chloridella.) 



Order 5. Cumacea. 



Two anterior thoracic legs are maxillipeds. Eyes 

 sessile. Small shrimp-like forms. (Diastylis.) 



Order 6. Arthostraca. 



First and sometimes second thoracic segments 

 fused with the head. Eyes usually sessile. (Tal- 

 orchestia, Gammarus, Caprella, Porcellio.) 

 Class 2. Arachnoidea. 



Head and thorax fused and bearing six pairs of 

 appendages. Respiratory organs lamellate and 

 abdominal or replaced by tracheae. 



Subclass 1. Gigantostraca. 



With lamellate abdominal gills. Coxal joints of 

 legs used as jaws. Marine. (Limulus.) 



Subclass 2. Arachnida. 



Respiration by lamellate lungs or tracheae. Four 

 pairs of walking legs. Mostly terrestrial. 



Order 1. Scorpionida. 



Abdomen segmented, posterior portion slender 

 and very flexible, frequently ending in a sting. 

 Four pairs of lung-books. Pedipalpi chelate. 

 (Buthus.) 



Order 2. Pseudoscorpionida. 



Abdomen segmented, without slender posterior 

 portion or sting. Pedipalpi chelate. Respira- 

 tion by tracheae. tChelifer.) 



Order 3. Pedipalpida. 



Abdomen flattened and segmented. Pedipalpi 

 simple or chelate. Two pairs of lung-books. 

 (Phrynus.) 



Order 4. Solpugida. 



Body composed of three segments. Chelicerae 

 chelate. Pedipalpi elongate, simple. Respira- 

 tion by tracheae, (Galeodes.) 



