They are not particularly distinctive morphologically, appearing in host tissues as enlarged spheres or ovals containing spores, and most were originally classified in various groups of fungi, protozoa, and algae. However, they form a coherent group on molecular trees, closely related to both animals and fungi and so of interest to biologists studying their origins. In a 2008 study they emerge robustly as the sister-group of the cladeFilozoa, which includes the animals.
Eukaryota tree. Note "Ichthyosporea" at bottom left, in Opisthokont clade. "Metazoa" are animals, and Choanoflagellates are closely aligned. Fungi is at other end of Opisthokont clade, with Cristidiscoidea closely aligned. Ichthyosporea is in the middle ("Meso-") of the fungi ("-myceto-") and the animals ("-zoea").
Since other new members have been added (e.g. the former fungal orders Eccrinales and Amoebidiales), Mendoza et al. suggested changing the name to Mesomycetozoea, which refers to their evolutionary position. Note the name Mesomycetozoa (without a second e) is also used to refer to this group, but Mendoza et al. use it as an alternate name for the phylum Choanozoa.