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ANIMAL KINGDOM Page 77 ABBOTTSCOLLEGE PHYLUM PLATYHELMINTHES (FLAT WORMS) Has over 10 000 described species 70% are internal parasites of other animals. Platys=flat, Helmins=worm Flatworms are dorso-ventrally flattened (from top to bottom) And unsegmented Not entirely aquatic Examples include: Parasitic flukes eg tapeworms Free-living flatworms eg planarians found in water Dorso-ventrally flattened Unsegmented worms 4 PARASITIC FLUKES Liver fluke Tape worms FREE-LIVING FLATWORMS Planarians A common name given to many non-parasitic flatworms Difference between the free living and parasitic forms are the special adaptations to a parasitic way of life BODY PLAN OF FLATWORMS 7 Acoelomate One opening to the gut Bilaterally symmetrical Show cephalisation Triploblastic ACOELOMATE ONE OPENING TO THE GUT Flatworms have a digestive cavity with only one opening Digestive cavity is branched, carrying food particles 10 copyright cmassengale BILATERALLY SYMMETRICAL 11 CEPHALISATION Sensory organs- are concentrated at their front end so that they can register what lies ahead. They show Cephalization. Simple eyes and sensors of chemicals, currents and temperatures are found in the head of the free-living forms. TRIPLOBLASTIC TISSUE LAYERS Formation of the mesoderm between the ectoderm and endoderm, making these animals triploblastic. The mesoderm forms mesenchyme with reacts with the epithelium to form the organs, such as the excretory and reproductive organs. The ectoderm and endoderm of the body can become more specialized for locomotion and digestion respectively. With an organ-system level of organization 14 DEFINITIONS: ADD TO WORKBOOK Planarians: non-parasitic flatworms Acoelomate: an invertebrate lacking a coelom and characterized by bilateral symmetry and a digestive cavity that is the only internal cavity Cephalization: a tendency in the evolution of organisms to concentrate the sensory and neural organs in an anterior head Mesoderm: is one of the three primary germ cell layers in the very early embryo Triploblastic: is a condition of the blastula in which there are three primary germ layers: the ectoderm, mesoderm, and endoderm Mesenchyme: is a type of undifferentiated loose connective tissue that is derived mostly from mesoderm, although some are derived from other germ layers. THE END