In the Gnathobdellidae the prevailing number of annuli to a segment is five; but here again the number is often increased, and Trocheta has no less than eleven.
This corre sponds to the usual pre sence (in the Rhyncho- bdellidae) of three annuli to each segment.
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.
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.
As remarked, the prevalent number of annuli to a segment is three in the Rhynchobdellidae.