Planarium
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Planaria sp., or cross-eyed worm, is a common freshwater flatworm. 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. Planaria are a member of the class Turbellaria and the phylum of flatworms, Platyhelminthes. They exibit 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 12mm, and the body has two eye-spots (also known as ocilli) that can detect the intensity of light. The eye-spots act as photoreceptors and are used to move away from light sources. 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.
The most frequently used in the laboratory is the brownish Dugesia tigrina. Other common varieties are the blackish Planaria maculata and Planaria dorotocephalia.
Organ Systems
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. The pharynx connects the mouth to the intestines. The intestines of the Planaria branch throughout the body. They eat living or dead small animals which 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. The undigested food leaves through the pharynx and mouth.
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. This means they are hermaphroditic, although they cannot fertilize themselves. 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 although it is not common in nature. If a Planaria is cut into two halves both halves may become two new Planaria.