What are intermediate organisms, from the standpoint of actin-myosin based locomotion?
. But: the Acoela have some features that are nearly as complicated as in other flatworms, especially the reproductive organs, and also some features of the nervous system and the presence of simple eyes. These are part of the reason for classifying them with other flatworms. They have some small muscle cells, of unknown function. They may have evolved from more complicated flatworms by loss of features. At best, they should just be thought of as a model that provides evidence that simple animals with these features are possible, and can survive as free-living organisms. Cellularization theories suggest that evolution proceeded from a large, multinucleate, ciliated or flagellated cell, by cellularization to a rather similar multicellular animal that also relied on cilia for locomotion. As long as they remained that small, locomotion by cilia was probably more effective than locomotion by contractile waves. However, to become large and fully exploit the advantages of multicellularity, l
