Nemerteans, often called ribbon worms or proboscis worms, are a diverse phylum of soft-bodied invertebrates. Approximately 1300 species are known, found across various aquatic environments. Their unique biological features make them a compelling subject for scientific study.
What Defines a Nemertean
Nemerteans are distinguished by their unsegmented bodies, which can range significantly in size. Some species are merely a few millimeters long, while others, like Lineus longissimus, can extend up to 30 meters, making them among the longest animals known. Most are 20 centimeters or less. Their bodies are cylindrical or flattened and exhibit bilateral symmetry.
They possess a complete digestive system with a mouth and a separate anus. Food enters through the mouth, moves through the foregut, and into the intestine. Their circulatory system is a closed loop of blood vessels, usually a pair of lateral vessels, a more advanced feature than in simpler worm phyla. Respiration occurs across their body surface, as they lack specialized gills.
The Proboscis: A Unique Hunter’s Tool
The eversible proboscis is a defining feature of nemerteans. This long, muscular tube is housed within a fluid-filled cavity called the rhynchocoel. It deploys rapidly through a pore by hydrostatic pressure. Once everted, it grasps and subdues prey.
The mechanism of the proboscis varies among nemertean groups. In some, like the Anopla, the proboscis exits from an orifice separate from the mouth and coils around the prey, immobilizing it with sticky, toxic secretions. Other nemerteans, classified as Hoplonemertea, possess a stylet—a sharp, pointed structure—on their proboscis. This stylet punctures the prey, injecting toxins that quickly paralyze or kill the victim.
Backup stylets are present in sacs, ready to replace a lost or damaged one. After capturing prey, a retractor muscle pulls the proboscis back into the rhynchocoel, drawing the prey towards the mouth.
Where Nemerteans Live and How They Feed
Nemerteans are mostly marine animals, inhabiting benthic (bottom-dwelling) or littoral (shoreline) environments. They are found buried in sediment, under rocks, or among plant life. While most are marine, some species have adapted to freshwater habitats, mainly in tropical and subtropical regions, and a few live in moist terrestrial environments.
Their feeding strategies are diverse, though most nemerteans are carnivores. They prey on small invertebrates such as annelids (segmented worms), clams, and crustaceans. Some species scavenge, using chemical senses to locate dead or decaying organisms. A few nemertean species also engage in commensal relationships, living inside mollusk mantle cavities and feeding on microorganisms filtered by their hosts.