Alexander Petniiikevitch, 



+ four pairs of post-oral appendages. No maxillipeds. 

 One pair of tracheal spiracles only, on the head. Geni- 

 tal openings on third somite Symphyla 



Abdomen rudimentary. Genital openings on the 2 d joint of 

 legsof the 4th pair, or 3d and 4 th, or 2d, 3d and 4 th. Number 

 of post-oral appendages 5 — 7 pairs .... Pycnogonida 

 + abdomen not rudimentary. Genital opening never on 



legs 



Head and thorax fused together. Cephalic part with 

 I pair of post-oral appendages. Thoracic part with 4 pairs 

 of legs (the last 2 pairs absent in i family of Acari) 



Arachnida 



+ head not fused with thorax. Three pairs of post-oral 



appendages in the head. Thorax with 3 pairs of legs 



Hexapoda (Insecta) 



CLASS ARACHNIDA 



Arthropods with at least 3 prae-oral segments, with i pair of prae- 

 oral appendages modified as chelicera, with 5 pairs of postoral appen- 

 dages, the first of which is represented by the pedipalpi. Head fused 

 with at least i thoracic segment, usually with entire thorax, forming 

 a cephalothorax. Genital opening on first somite of mesosoma. 

 Nephridia modified as coxal glands. Legs typically with 7, pedi- 

 palpi with 6 joints. 



This class is composed of 15 orders. 



Key to the Orders of Arachnida. 



1. Opisthosoma with an unsegmented telson 2 



+ Opisthosoma without a telson, sometimes with a seg- 

 mented whip 4 



2. Post-abdominal segments fused. Abdomen with a large 

 genital operculum and 5 gill-books. Legs of the ist, 2d, 

 and 3d pair chelate Niphosuia 



+ post-abdominal segments free ; legs not chelate .... 3 



3. All post-oral appendages with gnathobases. 5 pairs of 



gill-books Gigantostraca 



+ only the anterior 3 pairs of post-oral appendages with 



gnatho-bases. Pedipalpi chelate. 4 pairs of lung-books 



Scorpiones 



4. In front of the cephalothorax a movable plate (cucullus) 



Ricinulei 



