A puffball is a member of any of a number of groups of fungus in the ivision Basidiomycota. The puffballs were previously treated as a taxonomic group called the Gasteromycetes or Gasteromycetidae, but they are now known to be a polyphyletic assemblage. Their distinguishing feature is that they have gasterothecia (gasteroid basidiocarps) in which the spores are produced internally; that is, the basidiocarp remains closed, or opens only after the spores have been released from the basidia. The spores of puffballs are statismospores rather than ballistospores, meaning they are not actively shot off the basidium. They are called puffballs because a cloud of brown dust-like spores is emitted when the mature fruiting body bursts. Puffballs and similar forms are thought to have evolved repeatedly (that is, in numerous independent events) from hymenomycetes by gasteromycetation, through secotioid stages. Thus Gasteromycetes or Gasteromycetidae are now considered descriptive terms (more properly gasteroid or gasteromycetes) and not valid cladistic terms.