INSECTS. yD aT | 
its division of the body. The blood is for the most part 
white. 
386. The respiration differs in the different classes. In 
the Crustacea, the class to which the Lobsters and Crabs 
belong, the respiratory organs are, like those of the Fish- 
es, fitted for the action of air mingled with water. They 
are really gills in various forms. But in the Insects and 
Spiders the respiration is aerial; that is, the respiratory 
organs are fitted for the action of air alone by itself. 
387. The symmetry of the body is even more complete 
in the Articulates than it is in the Vertebrates. In the 
latter there are many of the internal organs in regard to 
which the two halves of the body are not alike, as the 
heart, stomach, lungs, etc. But in the Articulates this 
symmetry is extended even to some of these internal 
organs. 
388. The Articulata are commonly divided into seven 
classes. 1. Insects. These have the three divisions of 
the body, the head, the thorax or chest, and the abdo- 
men. They have antennz or feelers on the head, three 
pairs of legs, and generally one or two pairs of wings. 
2. Myriapoda, the Centipedes. They have no separation 
of the body into thorax and abdomen. The head, how- 
ever, is very distinct. There are seldom less than twen- 
ty-four pairs of legs. The segments of the body are nu- 
merous and equal. 3. Arachnida, including the Spiders, 
Scorpions, and Mites, are characterized by the union of 
the head and thorax, by the very distinct separation of 
this cephalo-thorax from the abdomen, by the possession 
of four pairs of legs, and by their want of antenne. In 
all these three classes the respiration is aerial. 4. Crus- 
tacea, or Crabs, Lobsters, etc. The respiration is aquatic, 
like that of Fishes. They have from five to seven pairs 
of legs. The body is variously divided. In some the 
three parts are distinct, as in Insects; in others the ar- 
rangement is like that of the Arachnida; and in others 
still it is like that of the Myriapoda. The classes already 
