丝盘虫（Trichoplax）经常是被认为是现存“最简单”的自由生活的动物，在关于早期动物起源的争论中处在中心位置。它是一种盘状的“扁盘动物”，直径约1毫米，首次是在一个水族馆的墙壁上被发现的。

现在，丝盘虫的基因组已经完成测序和分析。这种动物保留了与刺胞动物和两侧对称动物最后共同祖先的很多特征，它们存在于距今超过6亿年前。但丝盘虫也还有决定发育模式和细胞类型的基因，这些细胞类型从未在该动物身上看到过，说明在其生命周期中也许还有尚未被发现的阶段。

推荐原始出处：

Nature 454, 955-960 (21 August 2008) | doi:10.1038/nature07191

The Trichoplax genome and the nature of placozoans

Mansi Srivastava¹, Emina Begovic^1,2, Jarrod Chapman², Nicholas H. Putnam^2,9, Uffe Hellsten², Takeshi Kawashima^1,3, Alan Kuo², Therese Mitros¹, Asaf Salamov², Meredith L. Carpenter¹, Ana Y. Signorovitch^4,5, Maria A. Moreno⁴, Kai Kamm⁷, Jane Grimwood⁸, Jeremy Schmutz⁸, Harris Shapiro², Igor V. Grigoriev², Leo W. Buss^5,6, Bernd Schierwater^4,7, Stephen L. Dellaporta⁴ & Daniel S. Rokhsar^1,2

Center for Integrative Genomics and Department of Molecular and Cell Biology, University of California, Berkeley, California 94720, USA
Department of Energy Joint Genome Institute, Walnut Creek, California 94598, USA
Okinawa Institute of Science and Technology, Uruma, Okinawa 904-2234, Japan
Department of Molecular, Cellular, and Developmental Biology,
Department of Ecology and Evolutionary Biology, and,
Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520, USA
Division of Ecology and Evolution, Institut für Tier?kologie und Zellbiologie, Stiftung Tieraerztliche Hochschule Hannover, Bünteweg 17d, D-30559 Hannover, Germany
Stanford Human Genome Center, Stanford University, Stanford, California 94304, USA
Present address: Department of Ecology and Evolutionary Biology, Rice University, Houston, Texas 77005, USA.

As arguably the simplest free-living animals, placozoans may represent a primitive metazoan form, yet their biology is poorly understood. Here we report the sequencing and analysis of the 98 million base pair nuclear genome of the placozoan Trichoplax adhaerens. Whole-genome phylogenetic analysis suggests that placozoans belong to a 'eumetazoan' clade that includes cnidarians and bilaterians, with sponges as the earliest diverging animals. The compact genome shows conserved gene content, gene structure and synteny in relation to the human and other complex eumetazoan genomes. Despite the apparent cellular and organismal simplicity of Trichoplax, its genome encodes a rich array of transcription factor and signalling pathway genes that are typically associated with diverse cell types and developmental processes in eumetazoans, motivating further searches for cryptic cellular complexity and/or as yet unobserved life history stages.