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Planarium
The planarian Schmidtea mediterranea
Scientific classification
Kingdom: Animalia Subkingdom: Eumetazoa Superphylum: Platyzoa Phylum: Platyhelminthes Class: Turbellaria Order: Seriata Suborder: Tricladida Family: Planariidae

Planaria (family Planariidae) are common freshwater, non-parasitic flatworms of the phylum Platyhelminthes, class Turbellaria. It should be noted that the term "planaria" is most often used as a common name. It moves by beating cilia on the ventral dermis, allowing it to glide along on a film of mucus. Some move by undulations of the whole body by the contractions of muscles built into the body wall. They exhibit an extraordinary ability to [regenerate] lost body parts. For example, a Planaria split length wise or crosswise will regenerate into 2 separate individuals. The size ranges from 3 to 12 mm, and the body has two eye-spots (also known as ocelli) that can detect the intensity of light. The eye-spots are used to move away from light sources. Planaria have 3 germ layers [ectoderm(the outside of the body, mesoderm (the middle part of the body), and endoderm (the inside part of the body)], but lack a true coelom(body cavity). Rather, they have a solid body (acoelomate). They have a single-opening digestive tract, consisting of one anterior branch and two posterior branches in freshwater planarians. Because of this three-branched organization, freshwater flatworms are often referred to as triclad planarians. They are active carnivious flatworms that feed on daphnia and midges.

Planaria are common to many parts of the world and reside in fresh water ponds and rivers. They are also commonly found residing on plants.

Planarian on the glass of an aquarium.

The planarian has very simple organ systems. The digestive system consists of a mouth, pharynx, and an intestine. The mouth is located in the center of the underside of the body. Digestive enzymes secrete from mouth to begin external digestion. The pharynx connects the mouth to the intestines. The intestines of the Planaria branch throughout the body. They eat living or dead small animals that they suck with their muscular mouth. From there, the food passes through the pharynx into the intestines and digesting of the food takes place in the cells lining the intestine, which then diffuses to the rest of the body.

Planaria receive oxygen and release carbon dioxide by diffusion. The excretory system is made of many tubes with many flame cells and excretory pores on them. Flame cells remove unwanted liquids from the body by passing them through ducts that lead to excretory pores where the waste is released on the dorsal surface of the planarian.

At the head of the planarian there is a brain under the eyespots. From the brain there are two nerve cords which connect at the tail. There are many transverse nerves connected to the nerve cords which make it look like a ladder. With a ladder-like nerve system it is able to respond in a coordinated manner.

A planarian can reproduce either asexually or sexually. In asexual reproduction the planarian detaches its tail end and each half regrows the lost parts. In sexual reproduction each Planaria gives and receives sperm. Planaria have both testes and ovaries. Eggs develop inside the body and are shed in capsules. Weeks later the eggs hatch and grow into adults. Planarians can also reproduce by regeneration. If a Planaria is cut into two halves, both halves may become two new Planaria (regeneration). In one experiment, T.H. Morgan found that a piece corresponding to ¹⁄₂₇₉^th of a planarian could successfully regenerate into a new worm. This size (about 10,000 cells) is typically accepted as the smallest fragment that could regrow into a new planarian.