AskDefine | Define planarian

Dictionary Definition

planarian n : free-swimming mostly freshwater flatworms; popular in laboratory studies for the ability to regenerate lost parts [syn: planaria]

User Contributed Dictionary



  1. Any of various freshwater flatworms, of the class Turbellaria.

Extensive Definition

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 is also the name of a genus within the family Planariidae. Sometimes, it also refers to the genus Dugesia. 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 planarian split lengthwise or crosswise will regenerate into two 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 act as photoreceptors and are used to move away from light sources. Planaria have three germ layers (ectoderm, mesoderm, and endoderm), and are acoelomate (i.e. they have a solid body with no body cavity). 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.
Planaria are common to many parts of the world and reside in fresh water ponds and rivers. They are also found commonly residing on plants.
The most frequently used in the high school and first-year college laboratories is the brownish Dugesia tigrina. Other common varieties are the blackish Planaria maculata and Dugesia dorotocephala. Recently, however, the species Schmidtea mediterranea has emerged as the species of choice for modern molecular biological and genomic research due to its diploid chromosomes and existence in both asexual and sexual strains. Recent genetic screens utilizing double-stranded RNA technology have uncovered 240 genes that affect regeneration in S. mediterranea. Interestingly, many of these genes are found in the human genome (see link below).

Anatomy and physiology


planarian in Arabic: البلاناريا
planarian in Italian: Planarium
planarian in Japanese: プラナリア
planarian in Norwegian Nynorsk: Planaria
planarian in Russian: Планарии
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