Ecology
Most Nemerteans are carnivores and many have voracious appetites. Some species such as Lineus corrugatus have very cosmopolitan diets eating anything they can find. In aquaria these species have been fed on cooked animal fats and tinned sardines in tomato sauce. Other species appear to have more specialised diets such that Paranemertes peregrina shows a strong preference for the annelid worm Platynereis bicaniculata but will feed on other polychaetes as well. Amphiporus lactifloreus however has only ever been observed to feed on a single species of crustacean, Gammarus locusta.
Nemerteans appear to detect there food either chemically, particularly this applies to already dead items, or by blind chance, even some species that possess both eyes and cerebral organs do not necessarily use them to locate prey when hunting, i.e. Paranemertes peregrina when hunting polychaete worms on mudflats at low tide. Carnivorous Nemerteans use their proboscis when hunting. In those species with unarmed proboscis, the proboscis is shot out of its cavity and wrapped around the prey item. Toxins may be injected into the prey's body through holes made by special structures on the proboscis called rhabdites. Those species with armed proboscis have special spines or stylets on the send of the proboscis, these stylets pierce the body of the prey and inject toxins, generally in these species the proboscis is not wrapped around the prey. In either case common prey items are normally quickly subdued (in less than a minute) and drawn towards the mouth by as the proboscis is returned to its sheath.
Most Nemerteans reproduce by sexual reproduction being either male or female, however some freshwater and terrestrial species are hermaphroditic and can be self fertilising. In a very few species the eggs are retained within the female and hatch as small copies of the adult to then be released into the world. The gonads are temporary structures only, developing during the breeding season and then degenerating again. Each ovary or testis has its own duct to the outside.
Normally the eggs are laid in gelatinous strings and the sperm released into the water near them so that fertilisation takes place outside of the females body. The development of the eggs can be either direct or indirect (direct development is where the egg grows into a smaller version of the adult, as in humans, indirect development is where the egg grows into a larvae, which then later undergoes metamorphosis to gain the adult form, as in butterflies). Indirect in development in Nemerteans occurs in 3 forms, the egg either becomes a pilidium larvae, a Desor larvae or an Iwata larvae. The pilidium larvae looks like an old Roman legionnaire's helmet with a tuft of cilia arising from the top. It also has a lot of shorter cilia which beat regularly to generate a currents of water towards the mouth. After a short feeding period this then undergoes a metamorphosis to become a small adult-form juvenile Nemertean. The Iwata larvae is found only in one species, Micrura akkeshiensis and is free swimming, the Desor larvae is characteristc of species such as Lineus ruber and does not swim. Neither of these larval forms feeds, and after a time of development they undergo a metamorphosis to a an adult-form juvenile.
A number of Nemerteans can reproduce asexually through fragmentation of the body. Nemerteans have a good ability to regenerate damaged tissue and this form of asexual reproduction is related to that ability. Smaller fragments of the parent animal may secrete a mucilaginous case to develop in, this is called a cyst.