![]() Many of the eggs are released in feces and find their way into a waterway where they are able to reinfect the primary snail host. The parasite infects various organs in the body and feeds on red blood cells before reproducing. Infection occurs when a human enters the water, and a larva, released from the primary snail host, locates and penetrates the skin. The disease infects an estimated 200 million people in the tropics and leads to organ damage and chronic symptoms including fatigue. Trematodes are responsible for serious human diseases including schistosomiasis, caused by a blood fluke ( Schistosoma). The primary host is almost always a mollusk. Trematodes have complex life cycles that involve a primary host in which sexual reproduction occurs and one or more secondary hosts in which asexual reproduction occurs. The trematodes, or flukes, are internal parasites of mollusks and many other groups, including humans. Most monogeneans are hermaphroditic, but the sperm develop first, and it is typical for them to mate between individuals and not to self-fertilize. The worms may produce enzymes that digest the host tissues or graze on surface mucus and skin particles. ![]() ![]() ![]() They have only one host during their life, typically of just one species. The monogeneans are external parasites mostly of fish with life cycles consisting of a free-swimming larva that attaches to a fish to begin transformation to the parasitic adult form. (credit a: modification of work by Jan Derk credit c: modification of work by “Sahaquiel9102”/Wikimedia Commons credit d: modification of work by CDC) \): Phylum Platyhelminthes is divided into four classes: (a) Bedford’s Flatworm ( Pseudobiceros bedfordi) and the (b) planarian belong to class Turbellaria (c) the Trematoda class includes about 20,000 species, most of which are parasitic (d) class Cestoda includes tapeworms such as this Taenia saginata and the parasitic class Monogenea (not shown). ![]()
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