Individuals are about 1 mm wide and only 27 μm high, are irregularly shaped, and consist of a total of about 2,000 cells, which are diploid (2n = 12).
A diploblast has no mesoderm. In a coelomate, the tissue lining the inner side of the body cavity arises from the same germ layer as the tissue lining the outer side of the body cavity. A pseudocoelom has the same functions as a true coelom. An acoelomate triploblast has no endoderm.
In a pseudocoelomate, the tissue lining the inner side of the body cavity arises from the same germ layer as the muscles. The digestive tract of a coelomate functions as a coelom. A diploblast has no mesoderm. A pseudocoelom has the same functions as a true coelom.
Platyhelminthes ceased to be recognized as true protostomes. All Platyhelminthes must have a well-developed lophophore as their feeding apparatus. The removal of the acoel flatworms (Acoela) from the Platyhelminthes allowed the remaining flatworms to be a monophyletic clade clearly tied to the Lophotrochozoa.
Pseudocoeloms arose early in animal evolution and evolved into coeloms. Only a coelom is fully lined with mesoderm tissue. A coelom is a true body cavity, whereas a pseudocoelom is a false coelom that is not fully functional. Only a coelom is fully lined with mesoderm tissue.