660 ZOOLOGY SECT. 



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 AXD 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 chelicera? and pedipalpi and 

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

 tion, when present, are usually either trachea? 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 pre-abdomen of seven segments, and a posterior, narrower 

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

 of a sting. There are small chelate chelicerre and large chelate 

 pedipalpi. A pair of comb-like pectines occur on the second 

 segment of the pre-abdomen. The organs of respiration are four 

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

 of the pre-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 pre- and post-abdomen, and not pro- 

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

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

 a system of trachea?. A pair of spinning-glands are present. 



This order includes the Book-scorpions (Fig. 540). 



ORDER 3. PEDIPALPIDA. 



Arachnida in which the body consists of unsegmented cephalo- 

 thorax and flattened abdomen of eleven to twelve segments. 

 The chelicene are simple, the pedipalpi simple or chelate, and the 



