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CHAPTER III. 



Class II. — Arachnida. 



The Arachnida, MacLeay [Arachnides, Lamarck andLa- 

 treille ; Arachnoida, Leach), compose a class of articulated 

 animals, the name of which is derived from Arachne, under 

 which the spiders, which are the chief species contained in 

 it, were known to the Greeks. The celebrated Cuvier, hav- 

 ing by his invaluable discoveries in the com})arative anatomy 

 of the invertebrated animals, established the propriety of 

 the removal of the Crustacea, as a class, from the apterous 

 insects of Linnaeus, Aranea and the remaining genera (with 

 the exception of Pulex) were shortly afterwards raised to a 

 similar rank by Lamarck, under the name of Arachnides, 

 having been previously regarded as a chstinct order by Fabri- 

 cius, under the name of Unogata, and by Latreille, under that 

 of Acephala. The class has been restricted by Latreille, 

 MacLeay, and most modern entomologists, to its present 

 extent. Thus constituted, the Arachnida are distinguished 

 by their comparatively small size, their bodies in general 

 being short and rounded. They consist of two parts only, 

 the cephalo-thorax and abdomen, the head being so inti- 

 mately united to the thorax, that scarcely the sHghtest traces 

 can be perceived of their union ; whilst in others, even the 

 separation of the cephalo-thorax from the abdomen is 

 almost equally imperceptible. Like the Crustacea, they are 

 destitute of \\-ings, and are not subject to those metamorphoses 

 which distinguish the true insects. 



The organs of respiration, upon which great stress has 

 been laid as affording some of the primary characters of the 



M 3 



