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 branchie or 
gills are external, and in the former znternal, the air en- 
tering by spiracles before it acts upon them?. 
It may not be amiss in this place to lay before you the 
principal points in which the Crustacea and Arachnida 
agree with Jnsecta, 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 antenne. Their nervous system also is 
not materially different, and they are both oviparous. 
They differ from them in having the greater insections 
of the body less strongly marked; in the greater num- 
ber of legs on the trunk, the anterior ones perform- 
ing the office of maxilla; in their eyes usually on a 
moveable footstalk; their palpigerous mandibles; and 
their four antennze 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 
usually in the middle of their thorax”. ‘They have too 
a kind of gizzard and liver, at least the Decapods*, and 
their respiration is by gills. Genuine insects terminate 
* Pirate XXIXWFic. 2. Treviraaus, ¢. i. f. 1. | 
® Cuvier Anat. Comp. iv. 407. © N. Dict. d Hist. Nat. ix. 190. 
