The New Student's Reference Work/Arthropoda

Arthrop′oda.  The largest sub-kingdom of animals, containing an immense number of species.  They are known by having an articulated body and jointed legs.  The group embraces four great divisions or classes:

1. Crustacea, including the lobsters, crabs, crayfish, shrimps and others.  They usually live in the water and breathe by gills.  The common pill-bug is an exception.
2. Arachnida, the spiders, daddy-long-legs, scorpions, mites and others.  In this class the head and thorax are united.  They have four walking legs and no antennæ. The representatives are air-breathers.
3. Myriapoda, the centipedes and thousand-legged worms.  They are air-breathers, with the head bearing antennæ and distinct from the thorax.  The latter forms with the other joint of the body a continuous line of segments from six to two hundred in number.  Each of these segments bear a pair of legs.
4. Insecta, the largest class, including all insects.  These breathe by air tubes distributed through the body.  They show great variety of form and structure.  The king crabs, the fossil trilobites and the interesting peripatus (see Insecta) are sometimes separated from the other Arthropoda into distinct classes.