GMAS Biology Practice Test

The fluid-filled body cavity lined with mesoderm is the coelom. This is a true body cavity because mesoderm forms the lining on both its inner and outer sides, separating the cavity from the gut and from the outer body wall. This arrangement allows organs to be suspended in fluid and move independently, and it often provides a hydrostatic skeleton in some animals. In contrast, a pseudocoelom is a cavity not completely lined by mesoderm, so it isn’t a true coelom. The endoderm forms the lining of the gut, while the ectoderm gives rise to the skin and nervous system, not to a fluid-filled cavity.