

XII] CLASSIFICATION 283 



Class III. ARACHNIDA. 



Arthropoda with no antennae and no true gnathites. Prosoma 

 of six appendage-bearing segments followed by an abdomen 

 (occasionally divided into mesosoma and metasoma) whose appen- 

 dages when present are usually plate-like. 



Order 1. Araneida. 



Abdomen soft, unsegmented. Four to six spinnerets, two 

 to four lung-books and in the former case two tracheae. 



Ex. Epeira. 

 Order 2. Phalangida. 



No waist between prosoma and abdomen, the latter region 

 segmented. Tracheae, no lung-books. 



Ex. Oligolophus. 

 Order 3. Aearina. 



No waist. Abdomen unsegmented. Tracheae, no lung- 

 books. 



Ex. Tyroglyphus, Tetranychus. 

 Order 4. Scorpionida. 



Abdomen divided into a mesosoma of seven segments and 

 a metasoma of five segments and ending in a post-anal 

 poisonous telson. Four pairs of lung-books. 



Ex. Scorpio. 

 Order 5. Xiphosura. 



Prosoma of shield-shaped outline with a broad margin. 

 Abdomen unsegmented. Gill-books. The telson forms a spine. 



Ex. Limulus. 



Class IV. PANTOPODA. 



No gnathites and no masticatory processes of any kind attached 

 to the appendages. All the appendages leg-like. No tracheae. 

 Several pairs of genital apertures. 



Ex. Nymphon. 



Class V. TARDIGRADA. 



Cuticle thin, no distinct sclerites, gnathites a pair of stilets 

 within the buccal cavity. Only four other pairs of appendages and 

 these are stumpy legs ending in claws. 



