80 
Guide to Arachnida. 
Table- 
cases 
Nos. 19-26. 
Class 3.— ARACHNIDA. 
The Arachnida, a class which includes such familiar animals 
as the spiders, scorpions, and mites, constitutes one of the main 
divisions of the Phylum Arthropoda. The earlier members 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 
branchiferous 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 have been entirely replaced by 
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 known as 
Eurypterines. 2. The Pycnogonida, or Pantopoda, a marine 
group of doubtful affinities. 
Table of Classification of the Arachnida. 
Class— AEACHNIDA. 
Sub-class 1— EUARACHNIDA. 
Division A. — Delobranchia. 
Order 1 . — Xiphosura (King-crabs). 
,, 2 . — Gigantostraca (Eurypterines — Eossil forms). 
