The theory of cellularization, also known as Ciliate-acoel theory, is one of the theories explaining the origin of the metazoans. It was first based on Ernst Haeckel

Ernst Haeckel

The "European War" became known as "The Great War", and it was not until 1920, in the book "The First World War 1914-1918" by Charles à Court Repington, that the term "First World War" was used as the official name for the conflict.-Research:...

's assumption that the earliest animals derived from ciliate

Ciliate

The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagella but typically shorter and present in much larger numbers with a different undulating pattern than flagella...

 protozoans. Haeckel later abandoned this idea which is revived by Hadzi in 1953. According to the theory, a multinucleated unicellular ciliate ancestor would give rise to organisms similar to modern turbellaria

Turbellaria

The Turbellaria are one of the traditional sub-divisions of the phylum Platyhelminthes , and include all the sub-groups that are not exclusively parasitic. There are about 4,500 species, which range from to in length...

n flatworm

Flatworm

The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...

s by cellularization of the external layer. Recent molecular and morphologic data add increasing evidence against this view, and the alternative colonial theory, also proposed by Haeckel in the 1870s is gaining widespread acceptance.