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CLASS ARACHNIDA. 



State briefly the General Features of the Arachnida. 



The head and thorax are united and constitute the- 

 prosoma which is covered by the carapace. The abdomen,, 

 with or without segments, is sometimes divided into a 

 mesosoma and a metasoma. The head, thorax, and abdomen 

 may be fused together. 



The prosoma generally bears six pairs of appendages ; 

 the first two pairs are the chelicerae and pedipalpi, the 

 other four pairs are usually walking-legs. There are no 

 antennae and no true gnathites or jaws ; but the bases of 

 the limbs are modified as gnathobases, for crushing the 

 food. The abdomen has (on the mesosoma) six pairs of 

 appendages, specially adapted for respiration. In the pro- 

 soma there is (in King-crabs, Scorpions, and Spiders) a 

 skeletal structure, the endosternite. 



Arachnids breathe by gill-books (i.e., plates with numer- 

 ous branchial lamellae between which the water circulates), 

 or by lung-books (i.e., insunk gills adapted for air-breathing, 

 with numerous laminae between which the air circulates) 

 or by tracheae ; certain spiders have tracheae as well as 

 lung-books. The sexes are separate (Spiders show sexual, 

 dimorphism). Scorpions are viviparous. 



Name the Principal Orders of the Class Arachnida. 



Order XIPHOSHRA. Carapace of prosoma wide and horseshoe-shaped, 

 segments of abdomen fused ; a long tail-spine. Prosoma bears 

 seven pairs of appendages ; the last pair of legs have spa tula to 

 processes for digging. First pair of abdominal appendages form a 

 double genital " operculum ; " it covers in the remaining append- 

 ages which are modified for breathing as giU-books. 

 Example, Llnmln^ (King-crab). 



Order ScoRriONiDA (Scorpions). Form elongate. Prosoma (six 

 segments) short; mesosoma (seven segments) long; metasoma 

 long and narrow with five segments and a terminal curved poison- 

 sting. The pcdi palps large and chelate, for seizing prey. The 

 3rd, 4th, 5th and 6th segments of the mesosoma have each a vent ral 

 pair of slit-like stigmata, openings of the lung-books. 

 Example, Scorpio. 



