A hoplonemertean attacking the crab Uca musica. A. The everted proboscis rapidly entwines the prey and the stylet injects toxins to immobilize the prey, and probably injects digestive enzymes at the same time. B. The struggle is over and the worm inverts its proboscis. C. The worm emerges and searches for a place to enter the crab, whose tissues will be drained in about an hour. (Illustration by Emily Damstra)
mainly on crustaceans. The proboscis is everted and the central armature (the stylet) is used to pierce and immobilize the prey. After inversion of the proboscis, the worm uses its head to probe among the crustacean appendages, seeking a place where it can penetrate the prey; eventually, the head is wedged past the opening and the anterior gut is everted into the opening. It is uncertain whether proteolytic enzymes are inserted through the stylet-produced hole in the exoskeloton—histology of central armature suggests this—but at some stage, enzymes are injected to dissolve the prey's body tissue. Free-living marine suctorial nemerteans appear to be food specialists feeding primarily on amphipods. There are some enoplan species known to feed upon barnacles, limpets, and polychaetes. There are also examples of macrophagus ho-plonemerteans that engulf the entire prey after paralyzing it with a blow by the stylet. Freshwater hoplonemerteans are known to feed on oligochaetes, unicellular organisms, insect larvae, and other crustaceans. Very little is known about the ecology of pelagic nemerteans, including diet and feeding behavior.
There is one group of parasitic enoplan nemerteans (family Carcinonemertidae) found among the egg masses of certain crab species that feed on the host's embryos. There are also commensal enoplans (in family Bdellonemertidae) that live in the mantle cavities of bivalves where they feed on plankton from the mantle cavity. Obviously, the proboscis is not used to capture prey and has been (perhaps secondarily) reduced in these species.
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