98 THE MOST PRIMITIVE METAZOA 



The Acoela are primitive 



(1) The Acoela organisation corresponds to that of a ciliate 

 that has developed cell walls. 



(2) The Acoela have no gut. 



(3) The Acoela are hermaphrodite and have internal fertilisa- 

 tion rather like the syngamy of the ciliates. 



(4) The Acoela often have a syncytial gonad, epidermis, repro- 

 ductive system and digestive system. This is a relic of the 

 original ciliate syncytium. 



(5) The digestive system of the Acoela can be derived from the 

 protozoan food vacuoles (Figs. 5 and 35). 



(6) The nephridial system can be derived from the protozoan 

 contractile vacuoles. The Acoela do not have flame cells. 



(7) The Acoela usually have a ciliated ectoderm. 



(8) The Acoela often have musculo-epithelial cells. 



(9) The Acoela have no basement membrane. 



(10) The Acoela have a central mouth like the ciliates. 



(11) The Acoela have a simple pharynx derived from a 

 stomodeum. 



(12) There are no distinct gonads. 



(13) The rhabdites are derived from the trichocysts. 



The Acoela and the Polycladida differ in their morphology and 

 development from the rest of the Turbellaria. Hadzi suggests that 

 the Acoela gave rise to the Rhabdocoelida and the Polycladida. 

 The Rhabdocoelida then gave rise to the Anthozoa by the loss of 

 their protonephridia, the reduction of their nervous system, the 

 simplification of their digestive system, the loss of accessory 

 reproductive organs and reduction of the mesoderm. The slime 

 glands of the epidermis of the rhabdocoels gave rise to the 

 nematocysts. 



The derivation of the Anthozoa from the Rhabdocoelida in this 

 way would be very surprising with no other parallel in the animal 

 kingdom, i.e. reduction and simplification giving rise to a whole 

 phylum of widely diverse and successful animals. This does not 

 mean that such a reduction is impossible; it just seems highly 

 improbable. It also seems unlikely that the polyclads gave rise to 

 the ctenophores. 



