The of most animal groups were probably similar to modern flatworms (platyhelminths). Approximately 13,000 species of flatworms are found on Earth today. They are grouped in five classes:
flukes (Trematoda) - mostly parasitic
tapeworms (Cestoda) - mostly parasitic
planarians (Turbellaria) - mostly free-living
Monogenea - mostly parasitic
Aspidocotylea or Aspidobothria - mostly parasitic
Among other diseases, the flukes cause schistosomiasis in man.
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