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CHAPTER XVI. 

 ARACHNIDA.* 



(351.) THE Arachnidans long confounded with INSECTS, and 

 described as such even by recent entomologists, are distinguished 

 by characters of so much importance from the animals described 

 in the last chapter, that the necessity of considering them as a 

 distinct class is now no longer a matter of speculation. In IN- 

 SECTS, the external skeleton presents three principal divisions, 

 the head, the thorax, and the abdomen : but in the spider tribes, 

 the blood-thirsty destroyers of the insect-world, the separation of 

 the head from the thorax, which, by increasing the flexibility, ne- 

 cessarily diminishes the strength of the skeleton, is no longer admis- 

 sible ; and the process of concentration being carried a step fur- 

 ther, the head and thorax coalesce, leaving only two divisions of the 

 body recognizable externally, viz. the cep halo-thorax and the ab- 

 domen. Insects in their mature forms were found to be invariably 

 furnished with only six legs, but in the adult Arachnidans eight of 

 these limbs are developed. These characters in themselves would 

 be sufficient to discriminate between the two orders ; but when to 

 these we add, that in the Arachnidans the eyes are invariably 

 smooth, the antennae of insects represented by organs of a totally 

 different description, that the sexual apertures are either situated 

 beneath the thorax, or at the base of the abdomen, and, moreover, 

 that in the greater number of Arachnidans, respiration is carried on 

 in localized lungs (pulmonibranchia), instead of by tracheae as in 

 insects, we need not enlarge further in the present place upon 

 the propriety of ranking the Arachnida as a separate class. These 

 animals may be grouped under three principal divisions ; the first 

 of which is evidently an intermediate type of organization, com- 

 bining many of the characters of the Insecta with the external 

 limbs and palpi of proper Arachnida. 



(352.) The ARACHNIDA TRACHEAREA, in fact, breathe by 

 means of tracheae resembling those of insects, which are so ar- 

 ranged as to convey air to every part of the system ; and we may 

 therefore suppose that their circulatory apparatus, as well as their 



a spider. 



