80 Guide to Arachnida. 



Class 3.— ARACHNIDA. 



Table- The Arachnida, a class which inchides sucli famihar animals 



^x^^^,^ .„ ^s the spiders, scorpions, and mites, constitutes one of the main 



jSlos.lJ-26. ... 



divisions of the Phylum Arthropoda. The earlier memhers of the 

 class led an aquatic life, and the middle region of the body, in 

 these forms, was furnished with large plate-like respiratory 

 appendages, suitable for breathing oxygen dissolved in water. 

 The King-crabs are the only surviving representatives of these 

 l)ranchiferous forms. The rest of the living Arachnids are almost 

 invariably terrestrial forms, and the respiratory lamellae have 

 either sunk below the surface of the body, and become adapted to 

 breathe atmospheric oxygen, or liave been entirely replaced l)y 

 tracheal tubes. 



In the more primitive forms three principal divisions of the 

 body can be distinguished. The dorsal plates of the first of these 

 (prosoma or " cephalothorax ") are fused to form a carapace, and 

 its appendages are six in number. The middle region of the body 

 (mesosoma) is nearly always fused with the posterior region 

 (metasoma), to form a single division (the opisthosoma or 

 " abdomen "). The mesosomatic appendages may number six, 

 but are often suppressed or reduced in number. In its primitive 

 form the metasoma consists of six distinct limbless somites and 

 a post-anal spine or sting. 



The class is composed of two divisions : 1. The Euarachnida 

 or Arachnida proper, which includes the Scorpions, Spiders, Mites, 

 etc., and also the King-crabs and the extinct forms know^n as 

 Eurypterines. 2. The Pycnogonida, or Pantopoda, a marine 

 group of doul)tful affinities. 



Table op Classification of the Arachnida. 

 Class— AKACHN IDA. 



Sub-class 1.— EUAEACHNIDA. 

 Division A. — Delobranchia. 



Order 1. — Xijiliosura (King-cral)s). 



,, 2. — GuinnioHtraca (Eurypteiines — Fossil forms). 



