A soft unsegmented worm that catches prey by firing a proboscis out of its head.
Nemerteans are their own phylum. The body is soft, unsegmented and round in cross-section rather than flat, and it stretches and contracts continuously, so a worm that measured ten centimetres while gliding may be half that when it stops. Colours run to orange, pink, red-brown and cream, and several of the larger Indo-Pacific species are longitudinally striped — Baseodiscus is the genus most often photographed on rubble slopes. The animal a nemertean is most often confused with is a polyclad flatworm, and the separation in the water is that a flatworm is thin, has a rippling margin and a pair of folded pseudotentacles at the front, while a ribbon worm is a plain cylinder with no margin to ripple. This entry is pitched at the phylum because a photograph will not settle which one it is; the characters that separate the groups are the arrangement of the proboscis apparatus and the stylet, and both are internal.
The proboscis is the reason for the phylum’s other name. It sits inverted in a fluid-filled cavity above the gut and is fired out through a pore at the front of the head by hydrostatic pressure, faster than the eye follows, wrapping around small crustaceans and polychaetes; in one large group it also carries a calcareous stylet used to stab and inject the prey. Most muck-site sightings are of a worm gliding on mucus over silt at night, or of one drawn out from under a piece of rubble that has been turned. Very little lives on a ribbon worm, but the phylum supplies commensals of its own: species of Carcinonemertes live on the gill chambers and egg masses of crabs, and are found by examining a berried crab rather than by searching open ground.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.
