A planarian is one of the many flatworms of the traditional class Turbellaria. It usually describes free-living flatworms of the order Tricladida (triclads), although this common name is also used for a wide number of free-living platyhelminthes. Planaria are common to many parts of the world, living in both saltwater and freshwater ponds and rivers. Some species are terrestrial and are found under logs, in or on the soil, and on plants in humid areas.
The triclads are characterized by triply branched intestine and anteriorly situated ovaries next to the brain. Today, the order Tricladida is split into three suborders, according to their phylogenetic relationships: Maricola, Cavernicola and Continenticola. Formerly, the Tricladida was split according to habitats: Maricola, which is marine; Paludicola which inhabits freshwater; and Terricola, which is land-dwelling.
Planaria exhibit an extraordinary ability to regenerate lost body parts. For example, a planarian split lengthwise or crosswise will regenerate into two separate individuals. Some planarian species have two eye-spots (also known as ocelli) that can detect the intensity of light, while others have several eye-spots. 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 (they have a very solid body with no body cavity). They have a single-opening digestive tract; in Tricladida planarians this consists of one anterior branch and two posterior branches.
Planarians move by beating cilia on the ventral dermis, allowing them to glide along on a film of mucus. Some also can move by undulations of the whole body by the contractions of muscles built into the body membrane.
Triclads play an important role in watercourse ecosystems and are often very important as bio-indicators.
The most frequently used planarian in high school and first-year college laboratories is the brownish Girardia tigrina. Other common species used are the blackish Planaria maculata and Girardia 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 the existence of both asexual and sexual strains. Recent genetic screens utilizing double-stranded RNA technology have uncovered 240 genes that affect regeneration in S. mediterranea. Many of these genes have orthologs in the human genome.
Anatomy and physiology
The planarian has very simple organ systems. The digestive system consists of a mouth, pharynx, and a gastrovascular cavity. The mouth is located in the middle of the underside of the body. Digestive enzymes are secreted from the mouth to begin external digestion. The pharynx connects the mouth to the gastrovascular cavity. This structure branches throughout the body allowing nutrients from food to reach all extremities. Planaria eat living or dead small animals that they suck up with their muscular mouths. Food passes from the mouth through the pharynx into the intestines where it is digested by the cells lining the intestines. Then its nutrients diffuse to the rest of the planaria.
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. Also, flame cells remove unwanted liquids from the body by passing them through ducts which lead to excretory pores, where waste is released on the dorsal surface of the planarian.
The triclads have an anterior end or head where sense organs, such as eyes and chemoreceptors, are usually found. Some species have auricles that protrude from the margins of the head. The auricles can contain chemical and mechanical sensory receptors.
The number of eyes in the triclads is variable depending on the species. While many species have two eyes (e.g. Dugesia or Microplana), others have many more distributed along the body (e.g. most Geoplaninae). Sometimes, those species with two eyes may present smaller accessory or supernumerary eyes. The subterranean triclads are often eyeless or blind.
The body of the triclads is covered by a ciliated epidermis that contains rhabdites. Between the epidermis and the gastrodermis there is a parenchymatous tissue or mesenchyme.