GENERAL CLASSIFICATION 147 



single fertilisation. Moreover, the arachnids are the first animals in 

 which we find a rudimentary circulation, for according to M. Cuvier 

 they have a heart, from the sides of which issue two or three pairs of 

 vessels. 



Arachnids five in the air hke insects which have attained their 

 perfect state ; but they undergo no metamorphosis, never have wings 

 or elytra (nor is this due to any mere abortion), and they generally 

 keep hidden or live in solitude, feeding on other animals or sucking 

 blood. 



In the arachnids, the method of respiration is the same as in the 

 insects, but this method is on the verge of changing ; for the tracheae 

 of arachnids are very hmited, and do not extend throughout the body. 

 These tracheae are reduced to a small number of vesicles, as we learn 

 from M. Cuvier again {Anatom. vol. iv. p. 419) ; and after the arachnids 

 this method of respiration does not recur in any of the succeeding 

 classes. 



This class of animals should be treated with rnuch caution : many of 

 them are venomous, especially those living in hot chmates. 



TABLE OF ARACHNIDS. 



Okder 1. — Abachnids with Pedipalps. 



No antennae, but only pedipalps ; the head fused with the 

 thorax ; eight legs. 

 Mygale. Phrynus. 



Spider. Thelyphonus. 



Scorpion. Trombidium. 



Chelifer. Hydrachna. 



Galeodes. Bdella. 



Harvestman. Mite. 



Trogulus. Nymphon. 



Eiais. Pycnogonum. 



Order 2. — Arachnids with Antennae. 



[Myriapods and a few insects. H. E.] 



Two antennae ; the head distinct from the thorax. 



Louse [Hemiptera. H. E.]. — 



Ricinus. Centipede. 



— Scutigera. 



Sil ver- fish [Aptera. H. E.]. Julus (millipede). 



Spring-taiL 



