Planaria is a type of flatworm belonging to the traditional class Turbellaria. They are free-living flatworms of the order Tricladida, although this common name is also used for a wide number of free-living platyhelminthes. Here are some key facts about planaria:

• Anatomy: 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.

• Movement and Feeding: 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. 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.

• Reproduction: There are sexual and asexual planaria. Sexual planaria are hermaphrodites, possessing both testicles and ovaries. Thus, one of their gametes will combine with the gamete of another planarian. Each planarian transports its secretion to the other planarian, giving and receiving sperm. Eggs develop inside the body and are shed in capsules. Weeks later, the eggs hatch and grow into adults. Planaria also have a fascinating ability to regenerate into two separate individuals after damage, or can attach themselves to substrate, pulling their body to cleave themselves into two parts which then regenerate.

• Regeneration: Planaria have phenomenal regeneration abilities, with some documented to regenerate from just 1/200th of a piece when separated from the main body. If the head is separated from the body, then the head piece will grow another tail, with the separated body growing another head.

• Habitat: Planaria are mostly free-living flatworms that occur in fresh water and are sometimes seen in large masses. Some species are marine, others are terrestrial. Some species are parasitic, i.e., they obtain nourishment from the body of another living animal.

• Clinical Research: Planaria share many of the chemicals known as neurotransmitters and similar genes found in higher organisms such as vertebrates, including humans. For this reason, they are being pursued in clinical research such as drug development for preliminary trials, in place of higher organisms such as rodents.

It is important to note that planaria can be considered a pest by the majority of aquarists, as they can quickly multiply in number and be dangerous to other members of an aquarium.