Nemerteans or ribbon worms are small, flat, acoelomate worms related to flatworms. They are abundant in the littoral zone of temperate oceans where they make tubes in mud and sand or crawl among the rocks and seaweeds. There are about 1500 species worldwide, most in the ocean, but some in freshwater habitats or humid soils in tropical and subtropical climates. Most are grey, brown or black but a few are brightly coloured or have patterns on their shiny bodies. They are worm-shaped of course, and are like flatworms in being flat, having no true or pseudocoelom and having a ciliated epidermis.
What makes nemerteans different enough to be in their own phylum is that they have are, in many ways, more complex than flatworms. For instance, nemerteans have a closed circulatory system and some even have blood containing haemoglobin. They also always have a complete digestive system running one way from mouth to anus and a unique proboscis, carried in a special chamber called a rhynchocoel. Their epidermis, muscles and nervous system are much more highly developed than the flatworms. Only their reproductive system is simple. Meglitsch (p. 205) says of Nemerteans that: “They show the avenues of adaptive improvements which have, on the whole, been followed by the main invertebrate phyla of the annelid-arthropod stem.” Nemertea, for instance, is the first phylum that has a definite circulatory system.
I sometimes see them in the early mornings after rains, trying to crawl across dirt paths or other open spaces. They are doomed when the sun comes out as they desiccate quickly, so I sometimes try to pick them up and get them to moister, shady soils. They are sticky and hard to pick up and they leave slime trails behind them. Some of them have complex geometric designs in yellow on their brown backs.
The nemerteans are divided into two subclasses. The Anopla have the mouth behind the brain and the nervous system just below the epidermis. The proboscis has no stylets and is an undivided tube. The Enopla have a mouth in front of the brain and the nervous system inside body wall muscles. The proboscis is divided into regions and can have stylets, or piercing organs. The proboscis was once considered to be part of the nemertean gut but embryological studies showed it arose separately and is more of a sensory organ, although it is also useful for prey capture. It is coiled up inside the rhynchocoel and can be stretched and everted to hold prey and then be withdrawn by retractor muscles.
Internally nemerteans are somewhat variable. The gut can be simple or in specialised species it can have fore, mid and hind gut regions corresponding with an esophagus, stomach, intestine and rectum. Like the flatworms, the gut is embedded in solid tissues so it cannot move freely and peristalsis is not possible. Food must be moved along by ciliary action instead. They have the beginnings of an excretory system with the development of primitive protonephridia. Most nemerteans lack a respiratory system but some pump water in and out of the foregut for oxygen extraction. If this occurs, the foregut area is richly supplied with blood cells to pick up the oxygen. The nervous system is more centralised and with a larger brain than the average flatworm.
Sexes are almost always separate and in the few hermaphroditic species, self fertilization is prevented by protandry, where the sperm develop first. Fertilisation is sometimes external with gametes shed into the water surrounding the male and female. In other species the sperm enter the ovaries and development takes place there, with the females giving birth to live young. Usually though the eggs are laid in mucous-covered bunches.
Nemerteans have been considered as degenerate annelids, but due to the similarity of the sense organs, nervous system and excretory organs to flatworms, Meglitsch considers them to be descendants of rhabdocoel turbellaria (a group of free living flatworms). However recent molecular studies have placed the nemerteans among the trochozoan coelomates such as annelids and molluscs rather than with the flatworms. Whatever the case, Meglitsch has this to say (p 211): “Whatever their origin, the flatworms and nemerteans are remnants of a stock of animals that must have had an enormous influence on the course of animal evolution. Bilateral symmetry, cephalization and the establishment of organ systems familiar throughout the animal kingdom appear to have begun with animals at this grade of organization. Within this group of animals, the major adaptive trends that have culminated in the bigger part of the animal kingdom appeared and made their initial impact on animal form.”
References: Meglitsch, P. 1972. Invertebrate Zoology. Oxford University Press. http://www.ucmp.berkeley.edu/nemertini/nemertini.html
