MYRIAPODA. 155 
CHAPTER XVI. 
MYRIAPODA. 
Crass III. Myrtapopa.—This class is an extremely small one, 
and includes only the Centipedes and the Millipedes. In all 
the Myriapoda the head zs distinct, and not amalgamated with 
the thorax. There zs no clear boundary-line between the thorax 
and the abdomen, both being composed of nearly similar seg- 
ments. The body (except in the genus Pauropus) always con- 
sists of more than twenty rings, and the hinder segments 
which correspond to the abdomen, always carry locomotive 
appendages, whereas the abdominal rings in Arachnida and 
Insecta are always destitute of locomotive appendages. One 
pair of antenne is present, and the number of the legs ts always 
more than eight pairs. Respiration is carried on by branched 
air-tubes or trachez. 
In most of their characters the Myriapoda closely resemble 
the true insects, with which, indeed, they are not uncommonly 
classed. The true insects, however, always have the head, 
thorax, and abdomen distinct from each other, and have never 
more than three pairs of legs. In most of the Myriapoda 
the young or “larve” are more like insects than the adult, 
since they have only three pairs of legs, or are altogether desti- 
tute of feet. In some cases, however, the young Myriapod, on 
escaping from the egg, possesses nearly all the characters of 
the parents, except that the number of body-rings, and conse- 
quently of legs, is smaller, and increases with every change of 
skin (“moult”). The class is divided into two leading families, 
represented by the common Centipedes and Millipedes. 
The Centipedes (fig. 75) are carnivorous in their habits, and 
the organs of the mouth are adapted fora life of rapine. In 
addition to the parts of the mouth proper, they have two pairs 
of “ foot-jaws,” of which the second is hooked and perforated 
