The planarian is any of the species of flatworms that comprise the taxonomic family Planariidae. The non-parasitic planarian is found in various places like lakes, streams, under rocks, and in cave rarely. It has diverse colors of a flat body, from black to white and the size is various too. Besides, it is famous for its ability that can regenerate entirely itself. It is hermaphrodite, and can reproduce both sexually and asexually. Also, it fertilizes eggs internally by copulation. Unlike the other invertebrates, it does not possess respiration and circulation organs. The planarian does not cause disease or harm the people. However, it is a predatory animal to the protozoans.
The small flatworm, planaria, is tiny, flat(because of no coelom), and leaf-shaped. The colors of its body are black, gray, white and brown.  It ranges from about 3mm to 12mm and .30cm to 2.54cm in length mostly. The planarian has the rounded head on the dorsal side. At the head of planarian, there is chemosensory organs and two eye-pots which called ocelli. The eye-spots are groups of cells and quite sensitive to the light(presence and direction),so that it can find the changes in light.
The planarian has bilateral symmetrical body that can be divided into equal halves. It has no coelom (body cavity), but consists of three cell layers: ectoderm, endoderm, and mesoderm. The ectoderm is outermost layer that helps the animal to move, but it also protects. The endoderm is the innermost layer that lines the digestive tract. The mesoderm between other layers develops many organs and systems. 
The planarian must move to find the food. There are two ways for locomotion for eating, gliding(beating by cilia) and creeping(muscular activity). The first way, creeping, is through muscular contractions. The contractions of muscle layers which is under epidermis cause large movements. In another way, it glides smoothly by beating the cilia. Cilia is on the lateral and dorsal surface, but not head side. Moreover, the ventral side of cilia is not beating during the locomotion. 
It has a single-opening digestive system with one anterior branch and two posterior branches, very similar to cnidarian's. The digestive system is composed of a mouth, pharynx, and the intestine. The mouth of planarian is at the center of ventral surface and the tube-like pharynx is located between the mouth and a branched intestine. Planarian feeds on the decayed and small animals, because it is zoophagous (even it has no teeth). It likes the protozoans as flagellates, ciliates, sporozoans, and nematodes. This kind of food is sucked up by the mouth and goes through pharynx into the intestine. The food is broken down into small pieces and digested completely. Then, the food diffuses to body and remained things are expelled by pharynx, because of no exit. Moreover, it moves and feeds at night chiefly. 
Because of its thin body, the oxygen and carbon dioxide exchanges occur by diffusion, through the skin. It has many tubes and pores for excretion. This tubes extend through the mesoderm to the flame cells which is located in the end of tubes. It contains some cilia that beat to keep the flow in tubes and takes the unessential things away from the body(Thomas Porch, p.457). This excretory system are helpful to keep its water balance by ridding excess water. 
The flatworm, planarian has different nervous system from other invertebrates, ladder-type nervous organ.  There is a small brain and two nerve cords. The brain of planarian under eye-spots consists of two ganglia, clusters of nerve tissues. Also, it is joined by commissure. The nerve cords extend from the brain by many transverse commissures connecting. 
The planarian lacks of respiration and circulation organ, because there is no blood vessels and blood.
The planarian is hermaphroditic animal(has testicles and ovaries), so it can reproduce both asexually and sexually. It has lots of testicles and only one pair of ovaries. The ovaries are located at the anterior of it and the testicles are connected to the penis.  It has fabulous ability to regenerate their lost parts of body completely based on the stem cells. Asexually, when the planarian cut the parts of its body like crosswise, each of them regrows as the first condition. Interestingly, when it is divided by one hundredth of body, it can also regenerate. Nevertheless, it has several problems, so that it doesn't regenerate sometimes.
Sexually, two planarians get and give the sperm to each other instead of asexual reproduction.  This specie mates with a partner by cross-fertilization. It places eggs in the body where the capsule is after mating. Then, the planarian releases these eggs and they go to the plants or a rock in water. About 2-3 weeks later, the eggs emerge from the capsule and become adults. (Thomas, p457)
The planarians live all throughout the world, especially in the sea and fresh water. Most of them, freshwater planarian, are found in moist areas such as rivers and lakes, but some are not. Land planarians (Family Geoplanidae) are terrestrial and prefer the dark place instead of moist areas, so that they inhabit under rocks, logs, soil, on plants, and in cave. When the other species like to be in all temperatures of place, the planarian prefers to be in cold temperature of place. By using its pharynx, cannibalic planarian feeds on the decayed animals, eat worms(especially earthworms), snails, and other small animals.
The scientists were interested in its regeneration and researched it deeply. They found out the stem cells called neoblasts that help the planarian to regenerate its entire body and its very good model to study about this cells. It is only one that can peoliferate. The stem cells of planarian represent in adult planarian and its similar to that of humans. In Phillip Newmark's experiment, he found out the population of stem cells decreased without the protein, so that it was unable to regenerate. they found out the stem cells that help the planarian to regenerate its entire body. 
- Taxonomy-Planarian wikispecies.
- Anaomy-planarian new world encyclopedia.
- Land planarianP. M. Choate, 1998.
- The planarian.
- Planarian Caroline E. Stringer,
- Stem cells The National Science Foundation, 2008.
- regeneration-8 heads of planarian youtube, 2008.
- regeneration video the sanchez laboratory.
- excretion-planarian Thomas porch and Brad Batdorf, 1997.