Myxogastria/Myxogastrea (myxogastrids, ICZN) or Myxomycetes (ICN)[1] is a class of slime molds that contains 5 orders, 14 families, 62 genera, and 888 species.

All species pass through several very different morphologic phases, such as microscopic individual cells, slimy amorphous organisms visible with the naked eye, and conspicuously shaped fruit bodies.

Although they are monocellular, they can reach immense widths and weights: in extreme cases they can be up to 1 metre (3 ft 3 in) across and weigh up to 20 kilograms (44 lb).

[7][8] Reclassifications encounter problems because the Myxogastriae are morphologically very plastic, which is to say susceptible to environmental influences; only a few characteristics are diagnostic for a small number of species.

[10] In the past, authors have unsuccessfully tried to describe a new taxon based on a small number of examples, but this leads to numerous duplications, sometimes even at genus level.

[10][11] Echinosteliida Stemonitida Physarida Liceida Trichiida Ceratiomyxida Protosporangiida Cribrariales Reticulariales Liceida Trichiida Echinosteliida Clastodermatales Meridermatales Stemonitales Physarales The following classification is based on Adl et al. (2005)[14] while the classes and further divisions on Dykstra & Keller (2000), who included the Myxogastria in Mycetozoa.

In this monocellular phase, the Myxogastria consume bacteria and fungus spores, and probably dissolved substances, and they reproduce through simple cell division.

[19] If two cells of the same type meet in this phase, they cross-fertilise to a diploid zygote through the fusion of protoplasms and nuclei.

These are bacteria, protists, dissolved substances, moulds, higher fungi and small particles of organic material.

[14] All of these remain part of a single cell, which has a viscous, slimy consistency, and may be transparent, white, or brightly coloured in orange, yellow, or pink.

[19] The cell has chemotactic and negative phototactic capabilities in this phase, meaning that it is able to move towards nutrients and away from dangerous substances and light.

The sclerotium is a hardened, resistant form composed of numerous "macrocysts", which enable the myxogastria to survive in adverse conditions, for example during winter or dry periods,[14] in this phase.

According to laboratory researchers, changes in humidity, temperature or pH value as well as starvation periods were thought to be the triggers in some species.

[11][19] The plasmodium or parts of the fruit bodies can be smaller than one millimetre, in extreme cases they are up to a square metre and weigh up to 20 kilograms (44 lb) (Brefeldia maxima).

[11] The Myxogastria are most commonly found in temperate latitudes, and rarely in the polar regions, the subtropics or tropics.

Few Myxogastria species are found in the tropics and subtropics, mainly because of the high humidity which prevents the necessary dehydration of the fruit bodies to permit spore dispersal and promotes infestation by moulds.

The most important microhabitat is deadwood, but also the bark of living trees (corticolous myxomycetes), rotting plant material, soil, and animal excrements.

The comprehensive group of the nivicol Myxogastria populate closed snow blankets, to quickly fructificate at exposure – for example during thaws – and release their spores.

[31]:  483  Various Nematodes have also been observed to be their predators; they attach their posterior portion on the cytosol of the plasmodia or even live within the strands.

The combination of plasmodia and bacteria can bind atmospheric nitrogen or produce enzymes which make possible the decomposition of e.g. lignin, carboxymethylcellulose, or xylan.

Due to their short lifespan and the fragile structures of the plasmodia and the fruit body, fossilisation and similar processes are not possible.

Two older taxa – Charles Eugène Bertrand's Myxomycetes mangini and Bretonia hardingheni from 1892 – are now considered dubious and are today often disregarded.

[35][36][37] Friedrich Walter Domke described in 1952 a 35 to 40 million year old find in Baltic amber of Stemonitis splendens, an extant species.

[35] In 2019 sporocarps belonging to Stemonitis was described from Burmese amber, considered to be of a mid-Cretaceous age around 99 million years old.

In 1729, Pier Antonio Micheli thought that fungi are different from moulds, and Heinrich Friedrich Link agreed with this hypothesis in 1833.

Elias Magnus Fries documented the plasmodial stage in 1829, and 35 years later Anton de Bary observed the germination of the spores.

De Bary also discovered the cyclosis in the cell for the movement, he saw them as animal-like creatures and reclassified them as Mycetozoa, which literally translates "Fungus animals".

[11] From 1874 to 1876, Józef Tomasz Rostafiński, a student of Anton de Bary, published the first extensive monograph on the group.

These were groundbreaking works about the Myxogastria, as was the 1934 book The Myxomycetes by Thomas H. Macbride and George Willard Martin.

[7] Other notable researchers were Persoon, Rostafinski, Lister, Macbridge, and Martin and Alexopoulos, who discovered and classified many species.