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 +. 
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 trachee, 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 
* Treviranus, ut supra, 48. For the nervous system of scorpions, 
see t.1. f.13; and for that of spiders, ¢. v. fi 45. 
