NOTE - This is not a detailed zoology lesson, but a general overview of the phylum Nemertea - nemerteans/ribbon worms/proboscis worms. Nemertea is a relatively small group of about 1,300 species. While most ribbon worms are marine, some species are found in freshwater environments or in moist terrestrial habitats, such as under rotting logs or in damp soil. Most ribbon worms are free-living, but a few are parasitic, and some live commensally inside the mantle cavities of mollusks. Marine ribbon worms primarily inhabit the sea floor, often among rocks and sediments, though some are pelagic and live in open water.
Ribbon worms have unsegmented, narrow, fragile, worm-like bodies that can be cylindrical or flattened. Their lengths vary significantly with the majority being less than 8 inches in length; however, some species can reach several meters, with a few growing to over 100 feet in length. Some ribbon worms are brightly colored or patterned, but the majority are dull in appearance.
Most ribbon worms are active predators that feed on worms, crustaceans, small fish, and other invertebrates, while some are scavengers. The name “ribbon worm” or “proboscis worm” comes from their unique proboscis, a long, hollow, muscular tube housed inside their bodies. When needed, the proboscis is everted from the anterior end of the body, often with remarkable speed. This mucus-covered structure is used to catch prey and for defense. The shape of the proboscis can vary-it may have a single point or branch out with many ends or proboscides, which look like a mass of worm-like formations. When seizing their prey, they thrust out the sticky proboscis which quickly ensnares the prey by wrapping around it. In some species, the proboscis is armed with a sharp, pointed stylet used to stab and hold prey. This stylet may be equipped with poison-secreting glands that inject toxins and digestive enzymes to subdue the prey. Once captured, the proboscis is retracted, pulling the prey toward the mouth to be swallowed whole or externally digested.
Traditionally, the phylum Nemertea was divided into two main groups: Enopla and Anopla. Members in Enopla are referred to as armed ribbon worms. They are characterized by the presence of a stylet on their proboscis and the mouth opens in front of the brain. Members in Anopla are referred to as unarmed ribbon worms because they lack this stylet, and the mouth opens below or posterior to the brain.
However, recent studies have led to a restructuring of this classification into several superclasses, classes, and orders, and further taxonomic changes may occur as research progresses.
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