Phylum Platyhelminthes

Note - this is not a detailed zoology lesson.
This video is on they phylum platyhelminthes which are the platyheminthes or flatworms. Traditionally, Platyhelminthes was broken up into 4 main groups: Turbellaria, Trematoda, Monogenea, and Cestoda:
• Turbellaria: turbellarians include flatworms such as planaria and relatives. Turbellarians are mostly free-living worms, but a few are parasitic or symbiotic. The majority of turbellarians are adapted as bottom-dwellers in marine or freshwater or they live in moist terrestrial environments. These animals have simple life cycles.
• Trematoda: trematodes are commonly referred to as flukes. Examples of trematodes are liver flukes, blood flukes, and lung flukes. Adult trematodes are usually flat and leaflike or teardrop in appearance. Some are wormlike. Most trematodes are between 1 and 10 millimeters long, but some can get to several centimeters long. They are structurally like turbellarians, but they differ in having one or more suckers with which to cling to their host. Trematodes are all parasitic flukes, and as adults they are almost all found as endoparasites living on the inside of their vertebrate host. Trematodes usually inhabit and attach to specific hosts and specific regions of a host’s body, such as the circulatory system, nervous system, digestive tract, urinary tract, reproductive tract, and respiratory tract. Trematodes have complex, indirect life cycles. They are referred to as digenetic parasites because they require at least 2 hosts to complete their life cycle. Spending their larval stage within the body of an intermediate host or hosts and their adult life within the body of a definitive or final host. Trematodes have 1 to 2 intermediate hosts. If the fluke has one intermediate host, then it is usually a mollusc, typically a snail. Vertebrates are almost always the final definitive host of trematodes.
• Monogenea: monogeneans are often referred to as gill flukes. Most members of Monogenea are ectoparasites living on the outside of their host, primarily of the gills, skin, or fins of fish; however, some monogeneans are endoparasites living on the inside of their host. Monogeneans are also known for invading hosts such as turtles, frogs, and even hippos. Monogeneans have a simple, direct life cycle with usually 1 host. Parasites that complete their life cycle with only one host are referred to as monogenetic parasites. Monogeneans attach to the host with hooks, suckers, or clamps and they will move about the body feeding on skin cells and mucus. Some species will remain permanently attached.
• Cestoda: cestodes are referred to as tapeworms. Tapeworms are long, flat, and ribbon-like. Adult tapeworms are mostly intestinal parasites of vertebrates. Tapeworms have suckers, hooks, or both on the head for attachment to the host’s intestine. This holdfast structure is called the scolex. Tapeworms have no mouth or digestive system and instead absorb their food directly through their long body wall from the host’s gut. They also lack sense organs and a brain. Similar to flukes, tapeworms have a complex, indirect life cycle and are digenetic parasites. With a few exceptions, all tapeworms require at least 2 hosts to complete their life cycle. They usually spend their larval stage within the body of an intermediate host and their adult life within the body of a different, final host. The body of a true tapeworm consists of a linear chain of segments. Each segment is called a proglottid. Each proglottid contains male and female reproductive organs that can produce up to 100,000 eggs. Proglottids at the end of the body are shed daily, leaving the host’s body, usually with the feces. The eggs from the gravid proglottids are released after they are passed in the feces. Tapeworms can produce millions of eggs each year. For the tapeworm’s life cycle to continue, its eggs must be ingested by an intermediate host. Invertebrates are often the intermediate host.
Note, the Platyhelminthes 4 group division of has been revised and is no longer recognized. The Platyhelminthes phylum is now broken up into 2 main groups: Catenulida and Rhabditophora.
• Catenulida: members are referred to as “chain worms”. These species usually reproduce asexually by forming tissues or buds that are meant to detach from the parent and grow into new animals. This process is called budding; however, sometimes the buds do not fully detach from the parent. The parent and attached buds resemble a chain and that is why they are called chain worms.
• Rhabditophora: This group contains the remaining species and most species in the Platyhelminthes phylum. The group includes all the parasitic flatworms in the groups Trematoda, Monogenea, and Cestoda. It also includes most of the free-living species in the group Turbellaria.

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