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Interesting Facts about Flatworms
[Kingdom: Animalia] [Subkingdom: Eumetazoa (multicellular)] [Flatworm Phylum: Platyhelminthes]
The phylum Platyhelminthes contains a group of soft-bodied flattened invertebrate animals that are closely related to annelid worms and some snails.
These simple bilaterians (e.g., organisms with bilateral symmetry) are found in a number of environments around the world, including some marine and freshwater systems.
Marine Flatworm Distribution and Habitats
Planarians are a group of free-living flat worm species that flourish in marine ecosystems.
But, some of the dorsoventrally flattened flatworms (Platyhelminthes) also exist:
- In some moist terrestrial areas (e.g. underneath damp leaf litter, soil, or rocks).
- Living as parasites inside other organisms (e.g. flukes (trematodes), and hermaphroditic tapeworms).
Marine flatworms are found in aquatic environments around coral reefs and rocky shorelines. But, they're usually hidden from sight inside crevices or underneath stony outcrops.
Others inhabit freshwater streams, ponds, and lakes. But the parasitic flatworms need a host body to live in, such as fish, humans, livestock, or snails.
Characteristics of Flatworms
Platyhelminthes are bilaterally symmetrical invertebrates that also belong to the group Lophotrochozoa. For that reason, some of their close relatives are:
- Annelid worms (includes some leeches)
- Brachiopods (e.g., lamp shells)
- Mollusks (clams, sea snails, and octopuses)
- Rotifers (freshwater zooplankton)
There are four (4) main types of flatworms classified within the phylum Platyhelminthes. Even though most of them rely on a host for their survival, planarians are free-living organisms that inhabit various types of aquatic and terrestrial environments.
- Flukes (classification Trematoda): Parasitic flatworms that infect various organs of their host, but particularly the blood, eyes, lungs, and liver.
- Monogeneans (classification Monogenea): Even though they are a distinct group of flatworms, the group also consists of parasitic flatworms and flukes.
- Planarians (classification Turbellaria): These free-living planarians are usually found in various bodies of freshwater, some marine environments, and humid terrestrial terrains.
- Tapeworms (classification Cestoda): Also parasitic flatworms and tapeworms that have a scolex (a specialised head bearing hooks and suckers for attachment to the digestive system of their host).
As their common name suggests, the body of a flatworm is flat. But, it is also soft and it can be divided into two identical halves (bilaterally symmetrical).
They have three layers of tissue, known as the ectoderm, mesoderm, and the endoderm. There is no "true" body cavity (acoelomate) and the digestive system is often incomplete (e.g. the same opening for the mouth and anus) - or entirely absent.
But wait - there's more:
Flatworms do not possess any specific system for efficient circulation and respiration. Instead, they use diffusion to exchange gases.
The nervous system is represented by a simple "ladder-like" mechanism that contains nerve cords and a primitive brain (ganglia).
What Do Flatworms Eat?
The main source of food for a flatworm depends on whether it is parasitic or free-living.
The free-living flatworms, such as the marine flatworms, are mostly carnivores that feed on dead organic matter and small animals (e.g. crustaceans, insect larvae, protozoa, and tiny worms).
Whereas, flukes and tapeworms (parasitic flatworms) generally rely on the tissues of their host for nourishment, such as pre-digested nutrients and blood.
Different Types of Flatworm Species
Aquatic Flatworm (Dendrocoelum lacteum)
Australian Flatworm (Australoplana alba)
Gold Rim Flatworm (Pseudoceros paralaticlavus)
Linda's Flatworm (Pseudoceros lindae)
New Zealand Flatworm (Arthurdendyus triangulatus)
Persian Carpet Flatworm (Pseudobiceros bedfordi)
Pork Tapeworm (Taenia solium)
Salt and Pepper Flatworm (Tytthosoceros nocturnus)
Starry Flatworm (Pseudobiceros stellae)
Tiger Flatworm (Pseudoceros dimidiatus)
Wandering Broadhead Planarian (Bipalium adventitium)
Xenoturbella Bocki
Yellow Papillae Flatworm (Thysanozoon nigropapillosum)
How Do Flatworms Reproduce?
Flatworm reproduction is chiefly hermaphroditic, which also means they can reproduce either sexually or asexually.
Sexual reproduction occurs between organisms that have both male and female organs. Hence, after two separate individuals exchange sperm, any fertilised eggs will start to develop into new baby flatworms.
Most of the free-living flatworms can regenerate through asexual reproduction. Any piece that gets cut in half is able to regrow through a process of regeneration into a complete new worm.
Threats and Predators
The average lifespan for flatworms (phylum Platyhelminthes) can be anywhere between a few weeks and even up to several decades. In fact some Schistosoma blood flukes can live for up to forty (40) years.
Even so, some of the different threats that the different species of flatworm can face include amphibians, aquatic insects, crustaceans, fish, and larger flatworms. Yes... they are cannibalistic creatures!
Flatworms also face a wide range of human-induced threats, such as loss of habitat, underwater garbage pollution, challenges with invasive species, and vernal pools (the drying up of small ponds and streams).