What is "macrostomida"

The Macrostomida are a taxon of small basal free-living flatworms ( Turbellaria, Platyhelminthes), which inhabit freshwater, brackish, and marine environments. The taxon name was coined by Josef Meixner (1924). There are currently about 260 named species in this taxon.

As is the case for many other taxa, the taxonomic rank of Macrostomida is not very clear.