DEFINITION OF THE TERM INSECT. 9 



scorpions than to any members of the class Insecta at 

 present known. This circumstance, perhaps, may seem 

 to throw some doubt upon the modern system of classi- 

 fication. 



I must further observe, that the assertion of Treviranus, 

 which appears to intimate that the respiration of the pul- 

 monary Arachnida is the same with that of the Crustacea, 

 is not quite correct, since in the latter the branchial or 

 mils are external, and in the former internal, the air en- 

 tering by spiracles before it acts upon them a . 



It may not be amiss in this place to lay before you die 

 principal points in which the Crustacea and Arachnida 

 agree with Insecta, and also those in which they differ. 



The Crustacea agree with Insecta in having a body 

 divided into segments, furnished with jointed legs, com- 

 pound eyes, and antennae. Their nervous system also is 

 not materially different, and they are both oviparous. 

 They differ from them in having the greater insertions 

 of the body less strongly marked; in the greater num- 

 ber of legs on the trunk, the anterior ones perform- 

 ing the office of maxillae; in their eyes usually on a 

 moveable footstalk ; their palpigerous mandibles ; and 

 their four antennae at least in the great majority. But 

 the principal difference consists in the internal organi- 

 zation and the fountains of vitality; for the Crustacea have 

 a double circulation, the fountain of which is a heart in 

 the middle of their thorax b . They have too a kind of 

 gizzard and liver, at least the Decapods c , and their re- 

 spiration is by gills, Genuine insects terminate their 



a Plate XXIX. Fig. 2. Treviranus, t. If. 1. 



» Cuvier Anal, Cowp, iy. -107- c A" Diet, d'Hist. Nat. ix, IPO. 



