Fruiting body a stalked or sessile sporangium or plasmodiocarp, rarely almost aethalioid. Stalk, when present, ranging from short and stout to slender and relatively long, grooved or smooth, calcareous or limeless and translucent. Hypothallus ranging from inconspicuous to contiguous for a group of fruiting bodies, membranous and with no apparent lime to conspicuously calcareous. Peridium consisting of one or two (rarely three) layers, the outermost layer calcareous. Columella present or absent. Capillitium usually consisting of calcareous nodes connected by hyaline threads, these attached to the base and to the peridium, the nodes sometimes forming a pseudocolumella. Spores in mass black or dark brown.
Physarum is the largest genus in the myxomycetes, with more than 150 species having been described (Lado 2001). Some of these are highly variable, even to the point of producing sporangia that are stalked in some fruitings and sessile to even plasmodiocarpous in others. Moreover, the colour of the lime present in the fruiting body is not always constant for a particular species and may vary in response to such factors as the substrate upon which fruiting occurs. As such, it is sometimes difficult to identify a specimen of Physarum to species although the genus itself is usually easy to recognise