The world-wide distributed bacterivores genus Alaimus was described by Dr. J.G. de Man in 1880 from the Netherlands (Type species = A. primitivus de Man, 1880). The Latin name Alaimus refers to mouthless, i.e. to the very small mouth cavity. It is not the only reduced morphological character present in this genus, as it also shows a smooth cuticle (no cuticular ridges), very small labial sensilla and pore-like amphids (posterior to the head region). The female genital system is opisthodelphic with a closed vulva and a narrow vagina. About 55 valid species are known at present and the genus is terrestrial (particularly in mosses) and can also be found in semi-aquatic (fresh water) environments.