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What people say makes us excited::
The simplest animals that are bilaterally symmetrical and triploblastic (composed of three fundamental cell layers) are the Platyhelminthes, the flatworms. Flatworms have no body cavity other than the gut (and the smallest free-living forms may even lack that!) and lack an anus; the same pharyngeal opening both takes in food and expels waste. Because of the lack of any other body cavity, in larger flatworms the gut is often very highly branched in order to transport food to all parts of the body. The lack of a cavity also constrains flatworms to be flat; they must respire by diffusion, and no cell can be too far from the outside, making a flattened shape necessary.
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Life without a coelom : The image at left is a fluke (possibly a species of Probolitrema). Flukes, like other parasitic flatworms, have complex life cycles often involving two or more host organisms. At right, a planarian (Dugesia). Planarians are free-living flatworms, and have a much simpler life history. They inhabit freshwater, and are carnivores (even without teeth) or scavengers. Most are less than a centimeter long. (Click on either of the pictures above for a larger image).
Flatworms were once divided into three groups. The mostly free-living Turbellaria include the planarian, Dugesia, shown above; these are found in the oceans, in fresh water, and in moist terrestrial habitats, and a few are parasitic. The Trematoda, or flukes, are all parasitic, and have complex life cycles specialized for parasitism in animal tissues. Members of one major taxon of flukes, the Digenea -- which includes the human lung fluke depicted at right -- pass through a number of juvenile stages that are parasitic in one, two, or more intermediate hosts before reaching adulthood, at which time they parasitize a definitive host. The Cestoda, or tapeworms, are intestinal parasites in vertebrates, and they also show anatomical and life history modifications for parasitism.
It now seems likely that the first two of these groups are paraphyletic; that is, they contain some but not all descendants of a common ancestor. Recent molecular studies suggest that the Platyhelminthes as a whole may even be polyphyletic, having arisen as two independent groups from different ancestral groups. If this latter view is correct, then most of the flatworms may belong to the Lophotrochozoa, a large group within the animal kingdom that includes molluscs and earthworms, while the rest belong near the base of animal diversity.
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Marine flatworms : The marine flatworms (polycladids) are the largest of the free-living flatworms, sometimes reaching lengths of 15 centimeters. Polycladids get their name from their highly branched digestive cavity. These individuals were photographed on a reef near the island of Guam. (Click on either of the pictures above for a larger image).
Platyhelminths have practically no fossil record. A few trace fossils have been reported that were probably made by platyhelminths (Alessandrello et al., 1988), and fossil trematode eggs have been found in Egyptian mummies and in the dried dung of Pleistocene ground sloth. Trematode larvae that parasitize molluscs may leave pits or thin spots on the inside of the shell, and these pits may be recognized on fossil shells. If the mollusc is irritated by the presence of trematode larvae, it may be able to surround them with layers of shelly material - and thus do parasites become natural pearls.
Images of a number of free-living and parasitic flatworms, from Rudolph Leuckart's 19th century zoological wall charts, are available (look for "Platodes" in the index). More detailed classification of platyhelminths is available from the Tree of Life at the University of Arizona.
To find out more about tapeworms and flukes that cause human disease, read this handbook published by the U.S. Food and Drug Administration, view these pages produced by the World Health Organization. The disease schistosomiasis, or bilharzia, is a serious health problem in many parts of the world; you can learn more about it from the World Health Organization.
The life cycles of the three primary species of human schistosomes are similar. The male and female worms average about 10 mm in length and live in the veins of the abdominal cavity. Here they mate and the females produce eggs. The adult worms can live 20-30 years and, depending on the species, and each female can produce several hundred eggs each day. The eggs escape from the body by penetrating the walls of the veins and small intestine or urinary bladder, and they are passed in the feces or urine. The eggs hatch in water, the first intermediate host (a snail) is infected, and cercariae are liberated from the snails. When humans come in contact with water containing cercariae, the cercariae penetrate their skin and they become infected. This occurs when the humans swim, bath, wash clothes, etc., in rivers and streams. After the cercariae penetrate the skin the immature worms enter the circulatory system and migrate to the veins of the abdominal cavity, and in about six weeks they reach sexual maturity.
As the eggs of the schistosomes penetrate the walls of the veins and the small intestine or urinary bladder, they cause a significant amount of damage to the tissues. The tissues hemorrhage, so blood often appears in the urine or feces. As the infection progresses the tissues become inflamed and fibrotic and unable to function normally. Many of the eggs produced by the female worms do not escape from the veins, but are swept up in the circulatory system and deposited in the host's liver. The liver responds to the presence of the eggs by encapsulating them in a fibrous granuloma. The damage to the small intestine (or urinary bladder) and liver accumulate over time and result in a chronic, disabling disease that can be fatal.
As with most trematode infections, diagnosis most often depends on finding the parasite's eggs. In the case of S. haematobium, eggs are most often recovered in the urine; eggs of the other two species are most often recovered in the feces.
Open Letter of Invitation
2004 Schistosoma Genome Network Meeting
9-10 August 2004
Parasitology and International Programs Branch,
National Institutes of Allergy and Infectious Diseases
Rockville , MD., USA
A Joint Filarial/Schistosome Genome Meeting sponsored by UNICEF/ UNDP / World Bank/ WHO special programme in Tropical Disease Research will be held at the Parasitology and International Programs Branch of the National Institutes of Allergy and Infectious Diseases, Rockville, Maryland, USA from August 9-10. There are limited funds to support travel and room and board during the meeting. These funds are primarily for participants from Disease Endemic Countries.
Those who wish to attend should contact Philip T. LoVerde, Co-ordinator for the Schistosome Genome Network as soon as possible at loverde@buffalo.edu. As funds are limited, all of the interested parties, especially those from developed countries, are requested to pay for their own travel costs. International flights are available either directly to Washington, DC area via three airports, Reagan National, Dulles, and BWI.
The meeting will start in the morning of the 9th of August and end late afternoon August 10th.
We expect to accommodate as many applicants as possible and look forward to a very fruitful and exciting meeting.
Philip T LoVerde, Ph.D.
Department of Microbiology
138 Farber Hall
School of Medicine and Biomedical Sciences
State University of New York
3435 Main Street
Buffalo NY 14214-3078
Phone: 716-829-2459
Fax: 716-829-2189
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