Planaria

Planaria is a small aquatic genus of flatworm. The term is also used for other similar flatworms. They are fresh water animals which move in a smooth wave-like manner. Planaria are free-living. They can live in fresh water or very damp soil, depending on the type. 

Planaria are hermaphrodite: they produce both eggs and sperm. To reproduce, a planarian releases eggs and sperm into the water, where the eggs get fertilized. Planaria can repair damage by regenerating any part of their body. 

Planaria are the one of most simple creatures to develop a nervous system. Their nervous system consists of  ganglia under the eyespots and a cord of nerves that run down the body, under the gut (ventral nerve cord). The three pairs of ganglia under the eyespots make a inverted U shape that have nerves sticking out to the outside of the head so that the head of the planarian is more sensitive than the rest of the body. If the light hits the animal near the eyespots, they usually move away into darker water. 

The digestive system of Planaria consist of a mouth a pharynx and a gut cavity. Its mouth is on the underside of the body. Digestive enzymes are released from the mouth onto the food to soften it, Then the half digested food is sucked up the pharynx and into the stomach. From there the nutrients diffuse into the rest of the body. 

There are no circulatory or respiratory systems. Oxygen enters and carbon dioxide leaves the planarian's body by diffusing through the body wall. Planaria do not have a skeleton. They have an extendable crumpled muscle that allows them to move.  Also, planarians have three layers of body-wall muscles that are constructed of circular muscle fibers as well as longer muscle threads.Watanabe, Kenji; Hidefumi Orii and Hideki Ito. 2012. Anatomy of the Planarian Dugesia japonica I. The muscular system revealed by antisera against myosin heavy chains. Zoological Society of Japan.  

Planarians have both ovaries and testes. To reproduce, they can have sex with another planarian; this is called sexual reproduction. Another way planarians reproduce is called asexual reproduction. This is when a planarian splits and regenerates the parts that are missing. However, asexual reproduction is uncommon.

 References 

Category:Flatworms