228 NATURAL HISTORY. 
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 body. 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- 
