Phylum Platyhelminthes

• General characteristics: bilateral symmetry, dorsoventrally flattened bodies, cephalization, acoelomate (no body cavity) three cell layers, gastrovascular cavity (stomach), proto-nephridia (”primitive kidneys”), hemaphroditic (male + female), mostly aquatic
• Examples: class turbellaria (planaria) (free-living), class trematoda (flukes) and class cestoda (tapeworms) which are parasites.
• Free-living flatworms exist as either carnivores or scavengers.
• Free-living organisms have a gastrovascular cavity connecting to a mouth via an extendible, muscular pharynx. Food collected in gastrovascular cavity is absorbed by epithelial cells and digested intracellularly.
• parasitic organisms feed on blood, tissue fluids, or cell fragments within a host. Most do not have a complete digestive system (because they absorb nutrients that is already broken down)
• Thin bodies allow for materials to diffuse between the three layers of tissue for respiration and circulation. For excretion, flame cells are present to remove excess water. These function similarly to a primitive kidney, and are distributed throughout the body.
• Free-living Platyhelminthes are the most primitive organisms to display cephalization. They have “brain-like” ganglia (a group of nerve cells that act like a primitive brain). They also have specialized “eye-like“ eyespots that can detect light.
• Free-living Platyhelminthes reproduce via asexual fragmentation and sexual reproduction through the transfer of gametes from one individual to the other. All individuals are hermaphrodites.
• Parasitic flatworms have much more complex life cycles, usually involving a secondary host animal in which sexual reproduction takes place.