Classification of Multicellular Animals - Coelom
We know earlier that the pseudocoel gave animals specific selective benefits. Among other things, this fluid-tilled space worked as a hydrostatic skeleton increasing the efficiency of burrowing. However, in pseudocoelomates, organs lay loose in the body cavity. To circumvent this drawback, coelom evolved within mesoderm. The result was that the new body cavity, that is the coelom, came to be lined by mesodermal layer (peritoneal layer). The various organs also came to be suspended in the coelom through the mesodermal layer called mesenteries. This enabled the body wall to become much more muscular, with better developed longitudinal and circular muscles. The body cavity so became a more efficient hydrostatic skeleton now. The alimentary canal also became more muscular and more specialised. Various organs became better arranged in a much more stable way, without interfering with one another. The mesenteries as well served as a better medium for positioning blood vessels to the respective organs. So, development of coelom has been a main step in the evolution of more complex and larger animals.