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Phylum Nemertea, also known as ribbon worms, might look like simple, soft-bodied invertebrates at first glance, but are actually among the most stealthy predators armed with a unique secrete weapon. Residing in mostly marine environments, these benthic animals are often found in the intertidal zone of oceans. Nemerteans exhibit a wide range of unsegmented body sizes with some species being only a couple millimeters long while others are over a meter long. One species in particular can grow up to 30m meters (100ft), making it the longest recorded invertebrate!
Most nemerteans are carnivores, characterized as particularly active predators due to a specialized structure known as the proboscis. The proboscis apparatus is surrounded by a fluid-filled chamber called a rhynchocoel that is used to rapidly eject the proboscis to capture prey. Many species have a barbed proboscis that often delivers a toxin into their prey while others simply wrap around their prey using a mucus layer to hold on. With over 1,300 identified species, these organisms are considered important benthic organisms that play key roles in maintaining many marine ecosystems.
Defining Characteristics
Defining Characteristics
Slim, elongated bodies that closely resemble a ribbon
Bilateral symmetry
Triploblastic body plan: endoderm, mesoderm, ectoderm
Largest species reaches over 30 meters long (100 feet!)
Moderate Cephalization: defined head region with sensory organs
Proboscis: specialized feeding structure
Rhynchocoel: fluid-filled chamber holding proboscis used to rapidly eject the proboscis
Most species are harpoon-like with barbs that often deliver a toxin to their prey
Some species simply wrap around their prey to capture
Complete digestive tract: separate openings for mouth and anus that allows for maximized digestive efficiency
Closed Circulatory System: blood is contained within vessels as opposed to freely thoughout the body
Simple nervous system
Reproductive Adaptations
Reproductive Adaptations
Eggs are laid by females: often in gelatinous strands or masses
Eggs fertilized by male sperm: usually takes place externally (internal in some species)
Larval Stage: Planktonic, independent pilidium larva that develops from egg in some species and swim freely throughout open waters
Metamorphosis: Pilidium larva undergoes dramatic morphological change into the adult worm body plan. Nematodes without a larval stage do not undergo this process.
Juvenile Stage: Worm settles on the benthic surface of ocean floors
Adult Stage: Full grown nemerteans often live predatory lifestyles, reproducing sexually to continue the cycle or asexually through fragmentation.
Where are they found?
Where are they found?
Reside in intertidal marine zones
Usually found hiding under rocks and burrowing in soft sediments
Part of benthos community
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