228 NATURAL HISTOEY. 



mentioned have articulated members. The remaining 

 three have none. 5. The Annelida, the Leech and Worm 

 tribe, having the segments, very indistinct. 6. Entozoa, 

 Intestinal Worms, in which the articulated arrangement 

 is still more indistinct than in the Annelida. 7. Rotifera, 

 or Wheel Animalcules, very minute animals, presenting 

 the articulated character quite indistinctly. 



389. I now pass to the consideration of Insects. This 

 class of the Articulata has an immense number of dis- 

 tinct species, surpassing in this respect every other de- 

 partment of the animal kingdom. Dr. Carpenter esti- 

 mates the sub-kingdom of Vertebrates as containing 

 30,000 species, a number which is exceeded by one sin- 

 gle order of the Insect tribe, the Beetles. And numer- 

 ous as are the known species of Insects, it is supposed 

 that the number of those which remain to be discovered 

 is far greater. 



390. The name Insect comes from the Latin word in- 

 seco, to cut into, and refers to the divisions or sections 

 of the animal's body. The intervals between them are 

 so abrupt that they appear as if made by a cutting opera- 

 tion. The sections or segments are usually thirteen or 

 fourteen in number. One is the head; the chest or 

 thorax has three ; and the abdomen nine. 



391. The respiration of Insects is peculiar. They have 

 no lungs in one particular part of the bodj^ Their lungs 

 may be said to be in all quarters of the body, for air is 

 admitted by various openings into tubes which traverse 

 here and there. It is thus that the blood of the insect is 

 acted upon by the air. These openings are generally 

 mere slits like button-holes; but often they have two 

 valves which open and shut like folding doors, and some- 

 times they have over them a sort of fine grating to keep 

 dust from entering. As Insects are exceedingly active, 

 they require comparatively a large amount of air. They 

 are strikingly in contrast with Reptiles in this respect. 

 These latter are so dull and slow that they need but lit- 



