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.
As remarked, the prevalent number of annuli to a segment is three in the Rhynchobdellidae.
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.