CLASS VI. 

 IN S E C T A. -EN TO MO LOGY. 



INSECTS are distinguished by having, like the higher 

 classes of animals, a visible trunk and limbs, which de- 

 termines the general form of the body, and furnishes 

 them \vith means of motion ; it is, however, neither a 

 skeleton, like that possessed by the superior classes, nor 

 a calcareous shell, such as distinguishes the mollusca, 

 but species of horn-like case or armor, inside of which 

 the muscles of motion and the soft parts of the animal 

 are enclosed. This is plainly to be seen on the most 

 superficial observation of the structure of the crab, 

 where the whole animal is enclosed in a species of solid 

 armor, which yet leaves the limbs separate and distin- 

 guishable. This mailed covering being removed, the 

 interior is found to consist of muscles, which are of great 

 power, and serve for the purposes of progression and 

 motion. The most of insects are found to possess the 

 following wonderful internal structure, nevertheless it 

 varies somewhat in the different classes : Those which do 

 not breathe through branchiee, of which the crabs are 

 the only type among the articulata, possess a number of 

 air tubes, or trachea, which ramify like roots in the 

 interior, conveying the air to every portion of the sys- 



