A pseudocoelomate is an animal like a roundworm or rotifer. These animals have a body cavity, but it is not completely lined with a mesoderm. The mesoderm is the middle layer of tissue and cells which composes body structures into muscles and systems. The lack of this structure causes pseudocoelomates to be classified separately from other animals classified as coelomates.
Modern molecular phylogenies have brought into question whether coelomates and pseudocoelomates are actually independent lineages.
Source:
