Placozoa (Trichoplax) — highly simplified free-living metazoans originating during the peak of marine biodiversity diversification. Placozoans are morphologically and functionally asymmetric organisms lacking muscles, a nervous system, and clear body symmetry. Despite their structural simplicity, they exhibit complex behaviors, including social interactions. Research on Placozoa and the establishment of this phylum has a long history, which we present here as a chronological timeline spanning from early studies of their biology and morphology to modern genomic and transcriptomic analyses. This review synthesizes recent advances in Placozoa biology and critically evaluates accumulated data. Based on this analysis, we identify the most promising future research directions for this enigmatic group.
Romanova et al. (Wed,) studied this question.