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The Nemertea, also called ribbon worms or proboscis worms, (and sometimes referred to as Rhynchocoela or Nemertini,) are a distinctive group of 1150 known species of mostly marine invertebrates, found world-wide (Gibson, 1995).
Nemerteans have an unsegmented body, thin and elongated with no differentiated head. Unlike flatworms, which they resemble in many ways, they have a separate mouth and anus, and a digestive tract that runs the full length of their body. Very distinctive is their eversible proboscis, which is stored in the body-length long fluid-filled rhyncocoel when not in use. When activated, it is forced inside out with hydrostatic pressure from body muscle contractions to wrap around its prey , often administering venom through rhabdites (holes). Some species (members of the order Hoplonemertea) are armed with a pointed stylet at the tip of the proboscis, which the animal uses to puncture and kill its prey. Since stylets are frequently lost or broken in hunting, and growing worms require larger stylets, the stylets are continually formed (in large epithelial cells) and stored so there is always one on hand to replace an old or lost one.
Nemerteans have more developed muscle than flatworms, and can contract their body to up to a tenth of their extended length. They use body muscles to locomote (and have cilia covering their epidermis and lots of gland cells for mucus production so they glide across surfaces). They also use body musculature to move food through their gut and blood through their circulatory system.
(Kozloff 1990; Brusca and Brusca 2003)