94 ARTHROPODA. 



rizedby the absence of distinct respiratory organs. Arctisca and 

 Pentastomidea are two very aberrant and dissimilar groups. 



Abdomen segmented. 

 Respiration by tracheae. 



Abdomen distinctly separated from the 



cephalotkorax SOLPUGIDEA. 



Abdomen not distinctly separated. 



Maxillary palpi didactyle CHELIFERIDEA. 



Maxillary palpi monodactyle PHALANGIDEA. 



Respiration by pulmonary sacs. 



A postabdomen terminating in a claw SCORPIODEA. 



No postabdomen PHKYNIDEA. 



Abdomen unsegrnented. 



Abdomen united to the cephalothorax ACARIDEA. 



Abdomen distinct from the cephalothorax ... ARANEIDEA. 



A vermiform body with four pairs of rudimen- 

 tary limbs ARCTISCA. 



A vermiform body, the embryos only with two 



pairs of rudimentary limbs PENTASTOMIDEA. 



Order I. SCOEPIODEA. (Scorpions.) 



Abdomen indistinctly separated from the cephalothorax, of 

 seven segments, with a six-jointed postabdomen having a curved 

 claw on the extremity of the last segment. Maxillary palpi 

 (pedipalpi) longer than the feet, terminating in a didactyle hand 

 with a movable finger. 



The head is furnished with a pair of chelicerae or mandibles 

 (supposed by some to be modified antennae), each having a mov- 

 able and a fixed claw. There are from six to twelve ocelli, 

 two of which are larger and approximate, and placed more or 

 less in the centre of the cephalothorax. The respiratory organs 

 " are four pairs of flattened sacs, which open externally by as 

 many stigmata, on the sterna of the four posterior free thoracic 

 somites." 



The curved claw or telson at the extremity of the abdomen is 

 pierced for the passage of a venomous fluid, from a gland placed 

 in the last segment. In both sexes there are peculiar comb-like 

 organs situated directly behind the last pair of legs ; their use is 

 unknown. Scorpions are viviparous, producing 20-60 at a time ; 

 the young are carried on the back of the female for about a 

 month. They live on insects, which they sting to death. 



