These constellations were arranged in three concentric annuli, the northern ones in an inner annulus subdivided into 60 degrees, the zodiacal ones into a medial annulus of 1 zo degrees, and the southern ones into an outer annulus of 240 degrees.
The jus annuli aurei, or right of wearing a gold ring, originally a military distinction, became a senatorial privilege, which was afterwards extended to the knights and gradually to other classes.
This corre sponds to the usual pre sence (in the Rhyncho- bdellidae) of three annuli to each segment.
So that a leech in which only twenty-seven segments are apparent by the enumeration of the annuli, separate ganglia, nephridia, lines of sensillae upon the body, really possesses an additional seven lying behind that which is apparently the last of the series and crowded together into a minute space.
The annuli into which segments are externally divided are so deeply incised as to render it impossible to distinguish, as can be readily done in the Oligochaeta as a rule, the limits of an annulus from that of a true segment.