Origin of Bilateria
Origin of Bilateria First meta zones are very likely to radial animals Believed that bilateral symmetry evolved in radical ancestor when it is started creeping There are several theories given by many scientists for the Bilateria symmetrical animals are as follows:- 1.Ctenophore polyclad Theroy :- Ctenophores provided a a bridge or intermediate forms between radiate coelonterate and ancestors and bilateral polyclad flat warms. Based on resemblance:- Oval flat and shaped with dorsal ventral differentiation Creeping on entire ventral surface Presence of two dorsal tentacles Centrally located ventral mouth Radiating, anastomosing nervous system Swimming larval stages of polycalds have eight ciliated lobes and comparable to 8 comb rows of ctenophore Limitations:- Cleavage is biradical in ctenophore but spiral in polyclad Acoel flatworms are now regarded to be a most primitive 2.Planuloid-Acoeloid Theroy:- Ludwig Von Graft form