The family Proalidae is a small group ofmonogonont Rotifera that was erected as the subfamily Proalinae of the Notommatidae by Harring & Myers (1924). It was given family rank by Bartos (1959). The Proalidae comprises predominantly free-living epiphytic-epibenthic and psammobiontic species, inhabiting the littoral zone of fresh, thalassic and athalassic waters, and damp terrestrial habitats. Some species live epizoic on invertebrates and others are endoparasites of algae or ectoparasites of invertebrates.
Despite the early discovery of most of the species, the study of the Proalidae has been neglected due to the difficult identification of preserved specimens. As a consequence their distribution is insufficiently known and many species probably await description. The status of several species is confused, due largely to the fact that descriptions and illustrations of earlier authors are poor and only partially accurate. These descriptions mainly relied on the shape of the body, foot and toes, which are often rather similar in many members of the different genera. Less weight was given to the highly species-specific structure of the trophi which are, it must be admitted, very minute in some genera (10-20~m), and difficult to study with even modern techniques.
A thorough revision of the Proalidae is actually han1pered by the above mentioned lack of adequate descriptions and all but inexistent type material. Moreover, apart from a few exceptions, most of the species are rare and collected infrequently, and comparatively few specimens were available for this study. It is, however, clear that only a comprehensive application of scanning electron microscopy for the study of trophi structure, will allow a better specific identification and understanding of the inter-relationship of the species.
Illoricate, rarely semiloricate. Body swollen, fusiform or vermiform. Head, trunk and foot usually clearly defined. Corona simple, mostly supraoral, made up ofthe oblique buccal field and the lateral parts of the circumapical band.
The trophi are malleate, virgate or a modification of one of these types. A hypopharyngeal muscle inserts on the floor of the mastax cavity and serves a pumping function. Eyespot(s) usually on brain, rarely frontal, lateral or absent.