Most nemerteans are long, slender and somewhat flat. They can be highly colored, with patterns and are from one inch to several feet long. Elastic creatures, nemerteans are equipped with eyespots and sensory grooves, covered with cilia and have a solid internal mass of tissue; no body cavity. Their nervous and excretory systems are likeden to flatworms, with a digestive tract with a mouth and anus.

These creatures are predators. Their proboscis can be forced out longer than their body length to entangle small annelid worms and crustaceans with lots of paralyzing mucus. When dormant, the proboscis lies in a fluid filled sac in the mouth. The wor, contracts the sac forcing the fluid to force out the proboscis.

Sexes are separate in most nemerteans. Some shed eggs directly into water, the rest into mucus masses. Fertilization is mainly external. The eggs become ciliated larvae before assuming the adult form. Mucus encrusted eggs usually develop into complete but smaller adult forms.

There are two classes, Anopla -- unarmed nemerteans -- and Enopla -- armed nemerteans. Anopla include many forms whose larvae and planktonic. Their sensory grooves consist of slits along each side of the head.