Classification - The flatworms, or Platyhelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates. Unlike other bilaterians, they are acoelomates (having no body cavity), and have no specialized circulatory and respiratory organs, which restricts them to having flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion. The digestive cavity has only one opening for both the ingestion (intake of nutrients) and egestion (removal of undigested wastes); as a result, the food cannot be processed continuously.
Diversity - There are some 3000 species varying in size from microscopic to 600 mm, and although most are marine some species have managed to inhabit humid terrestrial environments and move on a bed of mucus. Many species in this phylum have become parasitic and live on the surface and inside bodies of other animals including man. Some of these parasitic forms such as liver flukes still resemble a basic flatworm form whereas others such as the tape worm have a highly modified morphology with hooks on their heads and an ability to detach egg-bearing sections of their posterior body parts.
Life History - Flatworms are hermaphroditic (having both male and female sex organs) and they typically reproduce both sexually and asexually.
Ecological Roles - Free-living flatworms are found in almost every kind of environment, on land and in fresh and salt water. They eat a variety of foods, including plankton, carrion, earthworms, snails, insect larvae, and small crustaceans. The parasitic flatworms often display a complicated life cycle, which typically requires development in two or three hosts before completion.
- Many platyhelminthes show highly specific adaptations to internal host environments. Many monogeneans, for example, show a marked preference for a particular gill arch in a fish. The scolex (head) of certain tapeworms of elasmobranch fishes (e.g.,sharks, skates, and rays) is highly specialized and can satisfactorily attach only to the gut of a fish possessing a complementary structure.
Human Impacts - Examples of Human Impacts: When humans ingest raw or undercooked fish, they also ingest immature stages of the fluke. The immature stages migrate to the bile ducts of the liver where they mature. The mature stages may live within the bile ducts for 15-20 years. While there they cause damage of the liver and may cause death of the human host. Eggs are produced by the adult within the bile ducts. The eggs travel through the bile ducts to the digestive tract and pass out of the host in feces. The eggs are ingested by snails, and the cycle is repeated. Humans can also become infected with a human blood fluke. Immature stages of this fluke also live within snails. These larvae leave the snail and are present on the ground in areas of poor sanitation. The immature stages enter humans by burrowing through the skin of a human host. The immature flukes enter blood vessels and mature there. They can cause anemia and damage to the liver, bladder, and brain.
Group Community - This phylum includes three major subgroups of flatworms.
The Turbellaria are the only free-living group, meaning that they can be found wandering around in the general environment. Turbellarians are aquatic (water-living), crawling around mostly on surfaces at the bottoms of bodies of water. Planaria, a common animal in laboratory exercises, is a turbellarian. The subgroups within this subgroup are divided mostly based upon how their digestive cavities are constructed.
The Trematodes (sometimes called flukes) are a group of parasites. There are three subgroups within this subgroup, but two of them have relatively minor effects on the world - one is a parasite mostly on the outside of fish, the other found mostly inside mollusks such as snails. The third group, the Digenea ("2 reproductions," or "2 lives"), include a number of serious parasites of humans and other large animals. Some examples are covered below, with more details of their life cycles.
The Cestodes, or tapeworms, are also parasites. In their adult forms, these worms attach with a specialized "head" called a scolex to the lining of their hosts intestines and begin to lengthen by adding proglottids, segments whose entire purpose is sexual reproduction. Tapeworms in large animals can get quite long, with the segments farther back being progressively older. Reproduction occurs between mature proglottids, which then fill with eggs and either drop off to be passed or break apart, so the eggs will be passed. As adults, tapeworms tend to be very host-specific - they can only live in one or a few species of hosts - and often cause hardly any ill effects to the host. Many species can control how many individual tapeworms are present in a host, somehow preventing new ones from attaching when a few are already there. In their larval forms, however, tapeworms may be much more dangerous to the host, as described below.
Tapeworms have no digestive systems as adults, which makes sense for an animal that already absorbs several materials through its surface and lives inside an intestine. Interestingly, tapeworms surface cells often have the same sort of surface-increasing structures as the lining cells of the intestine.
Diversity - There are some 3000 species varying in size from microscopic to 600 mm, and although most are marine some species have managed to inhabit humid terrestrial environments and move on a bed of mucus. Many species in this phylum have become parasitic and live on the surface and inside bodies of other animals including man. Some of these parasitic forms such as liver flukes still resemble a basic flatworm form whereas others such as the tape worm have a highly modified morphology with hooks on their heads and an ability to detach egg-bearing sections of their posterior body parts.
Life History - Flatworms are hermaphroditic (having both male and female sex organs) and they typically reproduce both sexually and asexually.
- The majority of sexual reproduction is through cross-fertilization (where both individuals fertilize each other).
- The fertilized eggs are frequently stored for a period of time within the flatworm, and are either retained in the parent or laid as egg masses.
- Some freshwater flatworms produce special overwintering eggs, which are retained within the parent until spring.
- The first method is through budding; the buds grow out along the length of the parent's body, forming chains until they are ready to separate into new individuals.
- The second method is called transverse fission; the posterior half of the worm attaches itself to the substrate while the anterior half continues to move forward until the two halves pull apart. Each half regenerates to form a complete worm. A few species can actually fragment themselves into several pieces, each of which regenerates to form several small worms.
Ecological Roles - Free-living flatworms are found in almost every kind of environment, on land and in fresh and salt water. They eat a variety of foods, including plankton, carrion, earthworms, snails, insect larvae, and small crustaceans. The parasitic flatworms often display a complicated life cycle, which typically requires development in two or three hosts before completion.
- Many platyhelminthes show highly specific adaptations to internal host environments. Many monogeneans, for example, show a marked preference for a particular gill arch in a fish. The scolex (head) of certain tapeworms of elasmobranch fishes (e.g.,sharks, skates, and rays) is highly specialized and can satisfactorily attach only to the gut of a fish possessing a complementary structure.
Human Impacts - Examples of Human Impacts: When humans ingest raw or undercooked fish, they also ingest immature stages of the fluke. The immature stages migrate to the bile ducts of the liver where they mature. The mature stages may live within the bile ducts for 15-20 years. While there they cause damage of the liver and may cause death of the human host. Eggs are produced by the adult within the bile ducts. The eggs travel through the bile ducts to the digestive tract and pass out of the host in feces. The eggs are ingested by snails, and the cycle is repeated. Humans can also become infected with a human blood fluke. Immature stages of this fluke also live within snails. These larvae leave the snail and are present on the ground in areas of poor sanitation. The immature stages enter humans by burrowing through the skin of a human host. The immature flukes enter blood vessels and mature there. They can cause anemia and damage to the liver, bladder, and brain.
Group Community - This phylum includes three major subgroups of flatworms.
The Turbellaria are the only free-living group, meaning that they can be found wandering around in the general environment. Turbellarians are aquatic (water-living), crawling around mostly on surfaces at the bottoms of bodies of water. Planaria, a common animal in laboratory exercises, is a turbellarian. The subgroups within this subgroup are divided mostly based upon how their digestive cavities are constructed.
The Trematodes (sometimes called flukes) are a group of parasites. There are three subgroups within this subgroup, but two of them have relatively minor effects on the world - one is a parasite mostly on the outside of fish, the other found mostly inside mollusks such as snails. The third group, the Digenea ("2 reproductions," or "2 lives"), include a number of serious parasites of humans and other large animals. Some examples are covered below, with more details of their life cycles.
The Cestodes, or tapeworms, are also parasites. In their adult forms, these worms attach with a specialized "head" called a scolex to the lining of their hosts intestines and begin to lengthen by adding proglottids, segments whose entire purpose is sexual reproduction. Tapeworms in large animals can get quite long, with the segments farther back being progressively older. Reproduction occurs between mature proglottids, which then fill with eggs and either drop off to be passed or break apart, so the eggs will be passed. As adults, tapeworms tend to be very host-specific - they can only live in one or a few species of hosts - and often cause hardly any ill effects to the host. Many species can control how many individual tapeworms are present in a host, somehow preventing new ones from attaching when a few are already there. In their larval forms, however, tapeworms may be much more dangerous to the host, as described below.
Tapeworms have no digestive systems as adults, which makes sense for an animal that already absorbs several materials through its surface and lives inside an intestine. Interestingly, tapeworms surface cells often have the same sort of surface-increasing structures as the lining cells of the intestine.