Systematic problems associated with nemerteans at most phyletic levels result from the large number of inadequately described species. The importance of adopting a standardized approach to taxonomic descriptions of nemerteans is demonstrated by reference to the phylogenetic relationships of the Heteronemertea. An evaluation of the systematic significance of several anatomical characters leads to the suggestion that the primitive heteronemertean organization included a proboscis with three muscle layers (outer longitudinal, middle circular, inner longitudinal), a foregut with neither splanchnic muscles nor subepithelial glands, and a blood system not developed into a vascular plexus around the foregut. A new classification of the Heteronemertea proposed on the basis of proboscis morphology recognizes seven families: Cerebratulidae, Gorgonorhynchidae, Lineidae, Mixolineidae, Panorhynchidae, Polybrachiorhynchidae, Valenciniidae.