CUVIER, in his great divisions or branches of the animal kingdom, recognized a group Articulata characterized by having a bilaterally symmetrical body, composed of a series of rings or segments serially arranged.

An excellent example of the arrange- ment of these rings can be seen in the common earth or angleworm.

These rings make a hardened external skeleton, which at once forms a framework for the attach- ment of muscles, and also a protection for the internal organs.

Typically, there is found in each segment a portion of each of the more important organs of the body.

Just under the dorsal surface is found an elongated dorsal vessel, which represents the heart ; the intestine lies in the median line of the body, which it usually traverses from end to end, while the nervous system, consisting of a series of enlargements, called ganglia, connected by nervous cords, extends along the floor of the body.