Turbellarians (Planarians; Free-Living Flatworms)

Family: 
Various families in class Turbellaria (free-living flatworms) in the phylum Platyhelminthes (flatworms)
Description: 

Turbellarians have primitive body plans. There are many species, but all have some traits in common. In Missouri, most are tan, brown, black or gray (although they can be brightly colored elsewhere). They are all flat or ribbonlike. Their bodies are “bilaterally symmetrical”—they have a left and right side, a “back” and “belly” and a “head” (they are not “radially” symmetrical like starfish and sea anemones).

Their simple nervous system is concentrated at the head end, and often there are 2 earlike flaps on either side of the head. There are often 2 primitive eyespots on the top surface of the head—they sometimes appear “cross-eyed.” The mouth is located about midway down the underside of the body; this is the only opening to the digestive tract. During feeding, a muscular, tubelike pharynx extends out from the mouth and sucks food into the creature. Tiny hairlike cilia on the underside, a thin layer of secreted mucus and subtle muscular contractions account for their “gliding” movement.

Size: 
Length: varies by species; some barely reach ¼ inch; others reach 1 inch; others (rarely in our area) more.
Habitat and conservation: 
Turbellarians lack a respiratory and circulatory system, so they are extremely thin and require aquatic (or very moist) habitats. They can be found in ponds, lakes, streams, springs and even cave streams, usually in submerged leaf litter or under rocks. Their eyes and “brains” are incapable of forming pictures, but they are extremely sensitive to light and always prefer to be in the dark. They are easily kept in an aquarium, if provided the right conditions.
Foods: 
Unlike their parasitic cousins in the flatworm group (the tapeworms and flukes), most turbellarians are free-living, and most are carnivores, eating tiny aquatic invertebrates such as rotifers, small crustaceans and other worms. Because they have relatively good sense organs, they make surprisingly efficient predators. Some species eat detritus and/or dead meat, such as dead fish and snails. Some eat algae.
Distribution in Missouri: 
Statewide.
Status: 
There are many species of turbellarians in Missouri and in the world. One notable Missouri species is the pink planarian (Macrocotyla glandulosa), which has never been found anywhere except in Devil’s Icebox Cave at Rock Bridge Memorial State Park (Boone County). It, and 3 other Missouri cave planarians, are listed in our state as Species of Conservation Concern. If more research was conducted, they would likely be placed on state and federal lists of species in danger of becoming extinct.
Life cycle: 
Turbellarians are hermaphrodites, meaning that each individual animal possesses both “male” and “female” anatomy, and when two turbellarians mate, they inseminate one another. Some species lay eggs encased in round capsules on short stalks. Turbellarians are also famous for their ability to regenerate when cut into pieces, and some species reproduce by splitting their bodies into two of these “clones.”
Human connections: 
Since they breathe through their skin, flatworms are sensitive to water quality and serve as indicators of reduced oxygen and other changes in their habitat. Many people know planarians from watching them regenerate from cut pieces in biology class, a wonderful thing to witness.
Ecosystem connections: 
Most of us know that ecosystems are based on the tiny plants and animals that form the base of the “food chain.” Turbellarians form an important link between the microscopic animals they eat, and their own predators—small fish, crayfish and aquatic insects—which are big enough for us to “see.”