Pseudocoelom - Metazoa
The platyhelminths which do not have a body cavity surrounding the gut, have a solid type of body constitution. The mesoderm completely fills the space between body wall and alimentary canal in the form of a network of cells called parenchyma. Such animals are called acoelomates. In nematodes, there is a different situation:
Figure: Section of (a) acoelomates (b) pseudocoelomates
the mesoderm is confined to specific circumscribed regions within the body cavity, and this cavity is neither lined nor filled with mesoderm. The cavity is in fact a persistent blastocoel of the embryo and is called pseudocoel. The internal organs are free within the pseudocoel. The body cavity has no lining of peritoneum derived from mesoderm. Animals so constructed are called pseudocoelomates.