Nemertea characteristics |
| Body cylindrical anteriorly and flattened posteriorly |
| An eversible and retractable proboscis that lies in dorsal cavity |
| A ventral mouth, through gut and terminal anus |
| An epidermis with cilia and gland cells |
| Respiration by diffusion |
| A nervous system with brain and two or three longitudinal cords |
| Sexual and asexual reproduction |
| Marine and freshwater. |
| No segmentation of the body, e. g. as in Annelida |
Nemertea are commonly known as the ribbon worms or proboscis worms. They range in length from less than 0.5 cm to over 50 m. This is more than twice the average length of an adult blue whale. So at 50 m the ribbon worm is the world's longest animal.
There are about 1200 known species. They are found world wide, but most commonly in the temperate zones, along the European and mediterranean coasts. They are found mainly in shallow waters, beneath shells, stones, amongst algae, and burrowing in sand and mud. There are two U. K. freshwater species.
They are acoelomate carnivores using their eversible proboscis to grasp prey. The proboscis lies in a cavity that runs almost the entire body length (see Prostoma rubrum below), as well as catching prey it can be used in locomotion and in defence. They prey on worms, snails, small crustaceans and small fish.
Contraction of the muscles exerts pressure in the proboscis cavity causing the proboscis to evert; it is retracted by the action of a longitudinal muscle. The blood flow system is driven by body movements and the contractions of the blood vessel walls.
Nemerteans can have from two to 250 pigment-cup eyes. The one below has two eyes, and above six eyes.
There are two Classes; the Anopla with a simple proboscis; and the Enopla with a more complicated proboscis armed with stylets (see Prostoma rubrum above right, which in some can deliver toxic secretions), and Tetrasemma sp. a mud-dwelling species below.
Tetrastemma sp. can eat prey wider than itself, and wide prey shows up as a bulge in its body which gets smaller as digestion proceeds and the prey passes further down the body.
The bootlace worm, Lineus sp., is perhaps one of the more commonly known Nemertea. It looks just like its name suggests, and can be as much as 30 metres long, but is usually 5 - 15 m. It is iridescent purple, and if picked up or mishandled it will exude a thick, sticky, smelly mucus which is its defence against predation. It is usually found entangled under rocks and crevices, and easily breaks apart if you try to untangle it.
Regeneration from the fragments is simple, and is an alternative means of reproduction for this worm.
Many are brightly coloured.