8 DEFINITION OF THE TERM INSECT. 



mals. It results from the observations of the most pro- 

 found comparative anatomist of our age, M. Cuvier, that 

 the Crustacea and Arachnida differ from insects properly 

 so called, and particularly from those that are furnished 

 with wings, in having a complete system of circulation, 

 a different mode of respiration, and that they have a more 

 perfect organization. Influenced by these motives, both 

 Cuvier and Lamarck have considered them as forming 

 two classes separate from insects. Treviranus, led by 

 considerations founded on the organs of circulation, of 

 respiration, and of generation, is of opinion that spiders 

 and scorpions ought to form one class with the Crustacea : 

 he observes, however, that the nervous system of all three 

 is very dissimilar ; and that in an arrangement founded 

 on this circumstance, the organs of motion, and the ex- 

 ternal shape, even spiders and scorpions must be placed 

 in different classes a . 



It is to be observed with regard to the Arachnida of 

 the French school, that the class as laid down by them 

 includes several animals that have no circulation, and 

 breathe by trachece, of which description are the mites 

 {Acarus L.), and the harvest-men (Phalangium L.) &c. ; 

 and therefore it has been divided into two orders, Pul- 

 •monaria and Tracheana ; but if the definition from the 

 internal organization be adhered to, the latter should 

 either remain with the class Insecta, or form a new one by 

 themselves. Yet the animals that compose the Trachean 

 order of Arachnida, their external form considered, are 

 certainly much more nearly related to the spiders and 



a Treviranus, ut supra, 48. For the nervous system of scorpions, 

 see t. \.f. 13 ; and for that of spiders, /. \.f. 45. 



