A puffball is a member of any of several groups of fungi in the division 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. The distinguishing feature of all puffballs is that they do not have an open cap with spore-bearing gills. Instead, spores are produced internally, in a spheroidal fruiting body called a gasterothecium (gasteroid ('stomach-like') basidiocarp). As the spores mature, they form a mass called a gleba in the centre of the fruiting body that is often of a distinctive color and texture. The basidiocarp remains closed until after the spores have been released from the basidia. Eventually, it develops an aperture, or dries, becomes brittle, and splits, and the spores escape. The spores of puffballs are statismospores rather than ballistospores, meaning they are not actively shot off the basidium. The fungi are called 'puffballs' because clouds of brown dust-like spores are emitted when the mature fruiting body bursts, or in response to impacts such as those of falling raindrops. 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' and 'Gasteromycetidae' are now considered to be descriptive, morphological terms (more properly gasteroid or gasteromycetes, to avoid taxonomic implications) but not valid cladistic terms.