Nephridia always paired, rarely (Pontobdella) forming a network communicating from segment to segment; lumen of nephridia always intracellular, funnels pervious or impervious.
The conditions which favour the vitality, growth and multiplication of the typhoid bacillus are the following: the soil should be pervious; it should be permeated with a sufficiency of decaying - preferably animal - organic matters; it should possess a certain amount of moisture, and be subject to a certain temperature.
In Hirudo the funnel is not pervious and is composed of a large number of cells.
Low down on the leeward side it is dark red and fertile, but still too pervious to retain moisture well.
In Leptaspis it is formed into a closed cavity by the union of its edges, and encloses the flower, the styles projecting through the pervious summit.