Nemerteans are most often found in ponds with abundant plants and filamentous algae, or in the substrate along the edges of still waters. They are likely to have patchy distribution within these habitats.

Nemertea are the simplest organisms that have a circulatory system. They have a fleshy, unsegmented body and an eversible proboscis used for feeding. They can be difficult to identify if their proboscis is not extended and are often misidentified as worms (Oligochaeta).

Their proboscis is a unique organ that can be extended out of the body through a pore, and can be as much as 3 times the length of the body. It is used for defense or gathering and handling food, but is usually tightly coiled and compressed inside the body.

Most ribbon worms are marine and can grow to a few feet in length. It is thought that there is only one genus native to freshwaters in the U.S.; other genera have been introduced, mostly through the aquarium plant trade. They vary in color but are usually dull white or yellow when young and bright red or orange when mature. Identification of Nemertea is based primarily on internal body structures.