丝盘虫（Trichoplax adhaerens Schulze）是1883年由Schulze在奥地利Graz大学的水族馆发现的。目前在扁盘动物门（Placozoa）中仅发现此一种（或可能有2种）动物，是已知最简单的多细胞动物之一，属于后生动物，但不属于真后生动物。1971年由德国学者Grell创建此门。丝盘虫为扁平薄片状，直径不超过4毫米，体形经常改变，边缘不规则，无对称性，但有恒定的背腹面。虫体由上下两层细胞构成，无体腔，无消化腔，无神经系统。扁盘动物与多孔动物门的动物一样，与动物界其他动物之间缺乏关联性。动物界除了这两门动物和中生动物外，都归到真后生动物。被认为是最接近合胞体假说中预测的多细胞动物始祖。

Trichoplax adhaerens is the only extant representative of phylum Placozoa, which is a basal group of multicellular animals (metazoa). Trichoplax are very flat organisms around a millimetre in diameter, lacking any organs or internal structures. They have two cellular layers: the top epitheloid layer is made of ciliated "cover cells" flattened toward the outside of the organism, and the bottom layer is made up of cylinder cells that possess cilia used in locomotion, and gland cells that lack cilia. Between these layers is the fibre syncytium, a liquid-filled cavity strutted open by star-like fibres.