Trematoda

Trematoda

Trematodes and flukes are the common names for the flatworms in the class Trematoda, phylum Platyhelminthes. Trematodes have most of the same features as other classes of Platyhelminthes. They are acoelomate , unsegmented, bilaterally symmetrical triploblasts that are flattened dorsoventrally . Trematodes do not have a respiratory system, but do have a mouth and primitive digestive and excretory functions, although these are not developed into full organ system as in higher vertebrates like mammals. They have a primitive nervous system with a ganglion, or brainlike structure, in the head region.

All trematodes are parasitic, and most are endoparasitic, living inside other animals. Trematodes infest various organs in a wide variety of animals. Adult trematodes have two specialized suckers. One is an oral sucker that surrounds the mouth. The other is a ventral sucker in the middle of the body that helps trematodes hang on to their host. Trematodes have simple sensory organs around the mouth, but do not have some of the more complex sensory organs found in other flatworms, such as the eye spots of turbellarians. The mouth of trematodes is a muscular pharynx, and the larvae and adult stages can suck their food from their host by grabbing on with their powerful mouths. Trematodes expel undigested material through their mouth because they do not have an anus. Nitrogenous waste excretion and other osmoregulatory functions are preformed by the protonephridium, which consists of flame cells and tubule networks, all of which act as a primitive kidney.

Most trematodes reproduce sexually. Most species are hermaphroditic , but a few have separate sexes. Trematodes have complex life cycles that include eggs, free-swimming ciliated larvae, and adults. Different life stages pass through one or more hosts.

The two major subclasses of Trematoda are best distinguished by the differences in their life cycles. Trematodes in the subclass Aspidobothria have only a single host in their life cycle. Trematodes in the subclass Digenea have a complex life cycle. These flatworms always pass through a mollusk in the first stage of their life cycle, have at least one additional host (but may have more), and complete their life cycle in the body of a vertebrate. Most trematodes are in Digenea, more than nine thousand known species.

Trematodes are some of the most harmful parasites of humans. Blood flukes (Shistosoma ) infect more than two million people worldwide with a disease called as schistosomiasis. People become infected by working, bathing, or swimming in water containing mollusks such as snails that carry the flatworms. At the beginning of the trematodes' life cycle, fertilized flatworm eggs are passed with human feces into the water, where they infect the mollusk. Once the eggs find a mollusk host the eggs grow into larvae that infect humans when released from the mollusk. The larvae penetrate the skin of humans and migrate through the bloodstream to the liver where they mature into adult flatworms. The adult flatworms can migrate to other organs, and the accumulation of eggs in various organs causes symptoms including acute pain and diarrhea. The adult shistosomes ingest the red blood cells of their host, causing anemia . Schistosome larvae that infect birds can also cause problems for humans. Larvae free-floating in water are killed by an immune response when they enter human skin, but the decomposing larvae cause an infection that leads to the bumpy, itchy skin known as "swimmer's itch." Other trematodes can be a problem for humans because they infest domesticated vertebrates such as sheep and cows.

see also Phylogenetic Relationships of Major Groups.

Laura A. Higgins

Bibliography

Anderson, D. T. ed. Invertebrate Zoology. Oxford: Oxford University Press, 1998.

Barnes, Robert D. Invertebrate Zoology, 5th ed. New York: Saunders College Publishing, 1987.

Campbell, Neil A., Jane B. Reece, and Lawrence G. Mitchell. Biology, 5th ed. Menlo Park, CA: Addison Wesley Longman, Inc., 1999.

Purves, William K., Gordon H. Orians, H. Craig Heller, and David Sadava. Life: The Science of Biology, 5th ed. Sunderland, MA: Sinauer Associates Inc. Publishers, 1998.