The chamber is called the rhynchocoel, and it runs much of the body length. Muscular contraction raises pressure inside it, and the proboscis everts through a pore at the front of the animal, sometimes emerging faster than the eye can follow. In one nemertean lineage the proboscis carries a calcareous stylet used to stab prey and introduce toxins; in another, including Cerebratulus, it is unarmed and works by wrapping and adhesion.
The separation matters anatomically. The proboscis is not a modified pharynx and does not open into the gut. Prey is captured by the proboscis and then taken in through a separate mouth, so the animal has two distinct anterior openings.
The toxins are genuinely potent. Nemertean species have yielded neurotoxic pyridine alkaloids and peptides, and some have been examined for pharmacological interest. Ribbon worms are also, in commercial terms, a nuisance, since some prey on bivalves and crustacean eggs. Precisely how many nemertean species exist is unclear, because they are soft-bodied, hard to identify preserved, and poorly sampled in many regions.