ARACHNIDA. 577 



as the cephalothorax, and its upper surface is tuberculated, while on 

 its ventral aspect are seen the openings of six pairs of stigmata. 



3. Adelarthrosomata. — Under this name may be included 

 Arachnidans such as the " Book-scorpions " (Pseudoscorpionidce) 

 and the " Harvest-men " (Phalangidce), in which the abdomen is 

 more or less distinctly segmented, but is not clearly separated from 

 the cephalothorax, the two regions being of equal width and con- 

 joined together ; while the respiratory organs have the form of 

 tracheae. Various Tertiary forms of the Pseudoscorpions have 

 been detected — principally in amber — but these all belong to exist- 

 ing genera {Chelifer, Cher?ies, &c.) Of the Phalangida no other 

 fossil forms are known except those which occur in amber, and of 

 these a number of types, belonging to several genera, have been 

 described. 



4. Pedipalpi. — Under this head may be included the two groups 

 of the Scorpions (Scorpiodea) and the Phrynidea, in which the abdo- 

 men is segmented, with or without a "post-abdomen," and the 

 breathing-organs are in the form of pulmonary sacs. The Scorpions 

 are characterised by their compressed bodies, and by the clear separ- 

 ation of the cephalothorax from the long and segmented abdomen. 

 The abdomen proper consists of seven wide somites, of which the 

 first carries on its ventral surface the opening of the generative 

 organs, closed by a delicate lid or operculum (fig. 431, g) ; the 

 second carries a pair of peculiar comb-like appendages ("pectines "), 

 the precise function of which is not known ; and the next four 

 exhibit the oblique apertures ("stigmata") of the four pairs of pul- 

 monary sacs. The abdomen proper is followed by six narrower 

 segments, which constitute a "post-abdomen," and of which the 

 last (the " telson ") is hooked, and is converted into a poisonous 

 "sting." The cephalothorax is covered by a shield-like carapace, 

 the upper surface of which carries a variable number of simple eyes, 

 one pair of which is larger than the others, and is placed dorsally, 

 while the smaller eyes are marginal. The first pair of cephalic 

 appendages, corresponding with the " falces " of the Spiders, are 

 converted into nipping-claws (" chelicerae") ; the maxillary palpi are 

 very large and end in pincers ; and a partially divided lower lip is 

 present. The four thoracic segments carry the four pairs of walking- 

 legs. 



The Scorpions possess a resistant chitinous exoskeleton, readily 

 capable of preservation in the fossil condition (see p. 573). It is 

 also probable that some of the more ancient forms were littoral in 

 habit, which would account for the occurrence of their remains in 

 strata of marine origin. The Scorpions are the most ancient group 

 of the A?'achnida, being represented in the Silurian rocks of both 

 the Old and Xew Worlds. Various Carboniferous Scorpions are 



vol. 1. 20 



