19 (17) Cuticular penis sheath present. Penial setae absent 20 



19 (17) Cuticular penis sheath absent. Penial setae present or absent 22 



20 (19) Penis sheath long, cylindrical, with hooded end; 

 over eight times longer than wide (Fig. 33). . . 

 Limnodribis hoffmeisteri 



Figure 33. — Penis sheath. 

 20 (19) Penis sheath thimble-shaped to somewhat elongate; less than six times longer than wide. 21 



3 



21 (20) Penis sheath elongate with a cuticular hook laterally, 

 near the distal end (Fig. 34). Anteriorly setae shal- 

 lowly bifid (Fig. 35); posteriorly setae single-pointed 

 (Fig. 36) Tubifex longipenis 



Figure 34 (top). — Penis sheath. 



Figure 35 (bottom left). — Dorsal and ventral anterior 



setae. 

 Figure 36 (bottom right). — Dorsal and ventral posterior 



setae. 



21 (20) Penis sheath thimble-shaped, unornamented (Fig. 37). 



All setae bifid (Fig. 38) Tubifex pseudogaster 



Figure 37 (left) .—Penis sheath. 



Figure 38 (right). — Dorsal and ventral setae. 



22 (19) Anterior setae two per bundle, some blunt and some with trifid ends 

 (Fig. 39). Posterior setae one per bundle, bifid with short teeth which 

 diverge at an obtuse angle; upper tooth often reduced or rudimentary; 

 setal node characteristically asymmetrical (Fig. 40) Clitellio arenicolus 



Figure 39 (left). — Anterior seta. 

 Figure 40 (right). — Posterior seta. 



22 (19) Anterior setae rarely two per bundle, never with trifid ends. Posterior setae rarely 

 one per bundle, never with teeth diverging at an obtuse angle. Setal node more or 

 less symmetrical 23 



11 



