The phylum Platyhelminthes are the flatworms. They are dorsoventrally flattened and have bilateral symmetry.

Some flatworms are parasites, while others are free-living carnivores or scavengers.

Flatworms have only a mouth for both food and wastes.

The planarian is the most common free-living flatworm found in water or moist places. They are hermaphrodites producing both eggs and sperm, but they exchange sperm with each other during sexual reproduction. Planarians also reproduce asexually by fragmentation.

Flatworms include a large number of parasitic forms, some of which are extremely damaging to human populations. Examples of parasitic flatworms are flukes and tapeworms.

Tapeworms are divided into sections called proglottids that each have a complete reproductive system producing fertilized eggs. Tapeworms are hermaphroditic (one body having both sexual parts), and they fertilize their own eggs. Ripe proglottids with their eggs pass out with the host's feces.

Tapeworms anterior end is called the scolex and is modified with both hooks and suckers to attach to the host's intestines. Humans most often get tapeworms from infected, undercooked pork, beef or fish.

Flukes have complex life cycles that involve more than one host. A fluke causes Schistosomiasis, a disease that affects 250 million people world wide.

The most common flatworm in our rivers is the Planaria.

The Planarias have a great capacity of regeneration. They are very fragile and when they are broken, each one of fragments regenerates the part that it lack.

Another one of the species common in mountain streams with current forts is the Dugesia that has arrow form.

It is a very sensible species to the contamination of waters.

Just as the planarias also it has great capacity of regeneration, being able to generate the complete animal from a small fragment.

Dugesia

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