First metazoans were most likely radial animals.
Believed that bilateral symmetry evolved in radial ancestor when it started creeping.
| Ctenophore Polyclad Theory | Ctenophores provided a bridge or intermediate forms between radiate coelenterate ancestors and bilateral polyclad flatworms. Based on resemblance: Oval flattened shape with dorsoventral differentiation Creeping on entire ventral surface Presence of 2 dorsal tentacles Centrally located ventral mouth Radiating, anastomosing nervous system Swimming larval stage of polyclads have 8 ciliated lobes comparable to 8 comb rows of ctenophores Limitations Cleavage is biracial in Ctenophore but spiral in Polyclad. Acoel flatworms are now regarded to be the most primitive. |
| Planuloid- Acoeloid Theory | Ludwig Von Graft formulated the theory, elaborated by Hyman Places Acoel flatworms at the base of the Bilateria and suggests origin of Bilateria from a planuloid ancestor, through a Acoeloid form Hypothetical planeload ancestor was very similar to the planula larva of Coelenterates — elongated, radially symmetrical and without mouth or archenteron Exterior cells are ciliated or flagellated epithelia-muscular cells, solid mass o interior cells were digestive and reproductive There was a nerve net under the epidermis and the anterior pole had a nervous centre Some planuloid ancestors may have evolved a creeping mode of life. This resulted in flattening out of oral-aboral axis, causing development of a ventral mouth Hollowing out of internal mass formed an archenteron, and differentiation of anterior region produced a head with brain and sense organs |