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Flatworms are worms that appeared on Earth probably about 600 million years ago.
These animals have a generally flat body, hence the name of the group: flatworm (from Greek platy: 'flattened'; and helmin: 'worm').
Flatworms, which comprise around 15,000 species, live mainly in aquatic environmentssuch as oceans, rivers and lakes; are also found in humid terrestrial environments. Some live free, others parasitize diverse animals, especially vertebrates.
Measuring from a few millimeters to meters in length, the flatworms have incomplete digestive tractthat is, they have only one opening - the mouth - through which they eat food and eliminate feces; therefore, they have no anus. Some even have no digestive tract and live adapted to parasitic life, absorbing through the skin the food previously digested by the host organism.
Among the many examples of flatworms we will study the planarians, at tapeworms and the schistosomes.
Measuring about 1.5 cm in length, these flatworms can be found in streams, lakes and wetlands. They move with the help of eyelashes and feed on mollusks, other worms and carcasses of larger animals, among other examples.
In the anterior region of the body of the planaria are located the head and the sense organs: eyelashesstructures capable of detecting contrasts between light and dark but not forming images; auricular organs, lateral expansions of the head capable of perceiving taste and olfactory sensations, helping the animal in locating the food.
The body is flat dorsiventrally and has the mouth located in the ventral region of the body. The planarian's gut is quite branched and acts by digesting food and distributing it to other parts of the body.
The adult planarian is hermaphrodite, ie, it has both the female and male genital systems. When two planarians are sexually mature and meet, they can copulate.
After sperm exchange through the genital pores, the animals separate and the eggs are eliminated to the outside. Inside each egg, enclosed in capsules, an embryo develops into a young planarian.
The planarians have great regenerating power. By cutting the animal into a few pieces, each can give rise to an entire planaria. Observe the scheme below.
Check out the other members of the flatworm group and the diseases caused by them: