The Annulose animals are characterised by the possession of a body which is usually more or less elongated, and is always bilaterally symmetrical, instead of being radially disposed. Very commonly the body is divided into similar (homonomous) segments, which may be definite or indefinite, and are arranged along an antero-posterior axis. Lateral appendages may be absent or present, and when present, are symmetrically disposed. A nervous system is present, and consists of one or two ganglia placed in the anterior part of the body, or of a ventrally-placed double gangliated chain.
The association of the Scolecida with the normal Annulose animals renders necessary an exceedingly general, and therefore correspondingly vague, definition of the sub-kingdom Annulosa. The sub-kingdom may, however, be divided into the following three primary sections, each of which admits of being characterised in a sufficiently definite manner: