Origin of Bilateria

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 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 formulated this theory elaborated by Hyman 
  • Places acoel flatworms at the base of Bilateria and suggested the origin of bilateral from a planuloid ancestor, through a acoelid larva
  • Hypothetical planeload ancestor  very similar to planula larva of coelenterates elongated, radially symmetrical and without mouth or archenteron
  • Exterior cells are ciliated on flagellated  epithelia - muscular cells, solid mass interior cells  were digestive and reproductive
  • There was a nerve net under the epidermis and the anterior Paul had a nervous centre
  • Some planuloid ancestor may have evolved  a creeping mode of life
  • This is resulted in flattening out of oral a oral axis causing the development of ventral mouth
  • Hollowing  out of internal mass formed archentron and differentiation of anterior region produced a head with a brain in the sense organs


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