Flatworms (class Turbellaria) live in many water bodies, from cold mountain streams to mucky urban drainage ditches and wetlands. They include the planarians, which are familiar to many from regeneration experiments in school biology class. Live Turbellaria are quite delicate and will tear easily if you try to pick them up with tweezers. When preserved in alcohol, they tend to break down; their spongy remains are sometimes difficult to recognize as an animal, and their white pharynx (feeding tube) may evert or completely separate from the body.

Flatworms somewhat resemble leeches, but have a much simpler body. They are oblong and have no body segments or appendages. They may have prominent eyespots or lobes on their front end, and the head is often triangular or arowhead-shaped.