xi PHYLUM ARTHROPODA 611 



rudiments of the thoracic limbs appear a series of six pairs of 

 abdominal appendages (ap. II. VI.) ; the place of the first of these 

 is afterwards taken by the operculum ; the second develops into 

 the pectines. The four posterior pairs become aborted, though 

 they apparently have some relation to the development of the 

 book-lungs. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Arachnida are air-breathing Arthropoda, in which the body 

 is usually distinguishable into two regions cephalothorax and 

 abdomen. The cephalothorax bears sessile, usually simple, eyes, 

 two pairs of jointed appendages the chelicerse and pedipalpi and 

 four pairs of legs. There are no antennae. The organs of respira- 

 tion, when present, are usually either tracheae or book-lungs, but in 

 the Xiphosura take the form of book-gills. Heart and vascular 

 system are usually present ; the heart is tubular, like that of the 

 Insects. The sexes are nearly always separate, and there is usually 

 no metamorphosis. 



The class is divided into the following orders : 





ORDER 1. SCORPIONIDA. 



Arachnida in which the body consists of a continuous cephalo- 

 thorax and an abdomen, the latter consisting of an anterior 

 broader prse-abdomen of seven segments, and a posterior, narrower 

 post-abdomen of five, with a caudal spine in the form of a sting. 

 There are small chelate chelicerae and large chelate pedipalpi. A 

 pair of comb-like pectines occur on the second segment of the 

 prae-abdomen. The organs of respiration are four pairs of book- 

 lungs in the third, fourth, fifth and sixth segments of the prse- 

 abdomen. 



This order includes the Scorpions. 





ORDER 2. PSEUDOSCORPIONIDA. 



Arachnida in which there is a continuous cephalothorax, some- 

 times marked dorsally with two transverse grooves, and a broad 

 abdomen, not divided into prae- and post-abdomen, and not pro- 

 vided with a sting. The chelicerae are very small, the pedipalpi 

 similar to those of the Scorpions. The organs of respiration are 



system of tracheae. A pair of spinning glands are present. 

 This order includes the Book-scorpions (Fig. 502). 

 ORDER 3. PEDIPALPIDA. 



Arachnida in which the body consists of unsegmented cephalo 

 thorax and flattened abdomen of eleven to twelve segments. 

 The chelicerse are simple, the pedipalpi simple or chelate, the first 



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