External Sense Organs of Tubifex tubifex and Limnodrilus hoffmeisteri (Tubificidae)

Sense organs were viewed by SEM for Tubifex tubifex (Müller) and Limnodrilus hoffmeisteri Clap. Individual sense organs were broadly circular in shape; they were regularly distributed on the prostomium and were found in compact bands around the circumference of segments I-III for both species. L. hoffmeisteri had up to 15 cilia per organ, with cilia from 2 to 6 μm in length. Diameter of sense organs ranged from 2 to 6 μm and mean densities (±SE) were 2(± 0.1)/1000 μ m2 at the tip of the prostomium, 7(± 0.3)/1000 μ m2 at the posterior regions of the prostomium, and 8(± 0.4)/1000 μ m2, 6(± 0.1)/1000 μ m2, and 2(± 0.2)/1000 μ m2 for segments I, II, and III, respectively. T. tubifex possessed up to 24 cilia per organ. Cilia were from 2 to 7 μm in length. Diameters of organs ranged from 5 to 9 μm and mean densities were 8(± 0.7)/1000 μ m2 over the entire prostomium, 13(± 1.0)/1000 μ m2, 3(± 0.6)/1000 μ m2, and $<1/1000\ \mu \text{m}^{2}$ on segments I, II, and III, respectively.