ARACHNID A. 399 



from the head, and the segments of the abdomen carry legs. 

 As is the case with all the air-breathing Articulates, the re- 

 mains of Arachnida, though of considerable theoretical inter- 

 est, are of very rare occurrence as fossils. They will there- 

 fore be very briefly noticed here. Of the groups of the 

 Arachnida, the Mites (Acarida), the Harvest-spiders (Phal- 

 angidce), the Book-scorpions (Pseudoscorpionidce), the Scorpions 

 (Pedipalpi), and the true Spiders (Araneida), have all been 

 detected in a fossil condition. The three first groups require 

 little consideration here, being almost unknown except as 

 occurring in amber, which is a fossil resin of late Tertiary 

 age. The Scorpions and Spiders both appear to have come 

 into existence in the Carboniferous period, and some of the 

 forms which then existed do not appear to have been strik- 

 ingly different from living types. 



ORDER PEDIPALPI. The typical members of this order are 

 the Scorpions (Scorpionidce), in which the abdomen is distinctly 

 segmented, and not separated from the thorax by any marked 

 constriction. The respiratory organs are in the form of pul- 



Fig. 250 A recent Scorpion (reduced). The great nipping-claws of the Scorpion are not 

 legs, but are a development of organs belonging to the mouth. 



monary sacs opening on the under surface of the abdomen 

 by distinct apertures or " stigmata." The jaws (maxillae) 

 carry an enormously -developed pair of nipping -claws (fig. 

 250), and the antennae are also converted into chelae. The 

 head carries six, eight, or twelve simple eyes, and the last 

 joint of the abdomen (telson), terminates in a hooked claw, 

 perforated for the transmission of the duct of a poison-gland. 



