<P> However, this scheme was followed by a number of college textbooks and some general classifications, but is now almost totally abandoned as a formal classification . Indeed, as late as 2010, one author of a molecular phylogeny study mistakenly called this classification scheme the "traditional, morphology - based phylogeny". </P> <Ul> <Li> Coelomate animals or Coelomata (also known as eucoelomates--"true coelom") have a body cavity called a coelom with a complete lining called peritoneum derived from mesoderm (one of the three primary tissue layers). The complete mesoderm lining allows organs to be attached to each other so that they can be suspended in a particular order while still being able to move freely within the cavity . Most bilateral animals, including all the vertebrates, are coelomates . </Li> <Li> Pseudocoelomate animals have a pseudocoelom (literally "false cavity"), which is a fluid filled body cavity . Tissue derived from mesoderm partly lines the fluid filled body cavity of these animals . Thus, although organs are held in place loosely, they are not as well organized as in a coelomate . All pseudocoelomates are protostomes; however, not all protostomes are pseudocoelomates . An example of a Pseudocoelomate is the roundworm . Pseudocoelomate animals are also referred to as Hemocoel and Blastocoelomate . </Li> <Li> Acoelomate animals, like flatworms, have no body cavity at all . Semi-solid mesodermal tissues between the gut and body wall hold their organs in place . </Li> </Ul> <Li> Coelomate animals or Coelomata (also known as eucoelomates--"true coelom") have a body cavity called a coelom with a complete lining called peritoneum derived from mesoderm (one of the three primary tissue layers). The complete mesoderm lining allows organs to be attached to each other so that they can be suspended in a particular order while still being able to move freely within the cavity . Most bilateral animals, including all the vertebrates, are coelomates . </Li> <Li> Pseudocoelomate animals have a pseudocoelom (literally "false cavity"), which is a fluid filled body cavity . Tissue derived from mesoderm partly lines the fluid filled body cavity of these animals . Thus, although organs are held in place loosely, they are not as well organized as in a coelomate . All pseudocoelomates are protostomes; however, not all protostomes are pseudocoelomates . An example of a Pseudocoelomate is the roundworm . Pseudocoelomate animals are also referred to as Hemocoel and Blastocoelomate . </Li>

When is the coelom said to be a true coelom