worms
Planaria are flatworms and members of the Platyhelminthes phylum.
Planaria are often found in aquariums with uneaten food. The planaria won't hurt the fish, but they are a symptom of too much gravel containing too much uneaten food, and that is not good for fish. They require a food source, which means there must be excess food wastes in the tank to support them.
If examined closely have eyespots as well as protrusions from the sides of their heads. Although they do not harm fish, they love to feast on eggs, and therefore are dangerous if breeding egglaying fish.
A clean tank is the best defense against becoming overrun with Planaria.
Free-living, non parasitic flatworms are common in lakes, streams, ponds, and other freshwater habitats. Planaria, usually dark brown, greenish, or tan, are found in shallow water underneath submerged rocks or vegetation. They can glide over the surface of objects and are sometimes upside-down on the underside of water surface film. The body of Planaria is non-segmented and bilaterally symmetrical. The head is triangular shaped and contains two eyespots that detect light. Worms can shorten and change shape using muscle cells whose contractions are controlled by a primitive nervous system. Asexual reproduction allows a new head and tail ends to form by a process of tissue regeneration. Sexual reproduction is also possible after worms exchange sperm; worms are hermaphroditic. After internal fertilization, numerous zygotes are deposited into a small, dark capsule, called a cocoon, which is about 1 mm in diameter. The cocoon is attached to submerged rocks or plants and, after further development, small worms emerge from an opening in the cocoon. There is no larval form.
