Marine Phyla Pages -- Coastal Carolina University
NEMERTEA
Flukes & Cestodes
Nemertea
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Defining Characteristics
The nemerteans, also known as rhynchocoels, ribbon-worms and nemertina, include approximately 570 species of worm-like animals [Barnes, 1968]. The most characteristic feature is a projectile proboscis, which may be used like a lasso, or a grappling iron in prey capture.
Systematics
Ribbon worms are highly elongate in body form. They have a remarkably protrusible proboscis, which can be fired accurately to ensnare prey animals and is characteristic of the phylum as a whole. When not in use the proboscis is inverted in a sheath called the rhynchocoel, which lies immediately above the alimentary canal. These are truly triploblastic animals with a distinct ectoderm, endoderm and mesoderm. The ectoderm is ciliated, and is dotted with gland cells and rhabdites. Below the epidermis lie the circular and longitudinal muscle layers. Parenchymal tissues package the gut, rhynchocoel and associated blood vessels. The body is roughly cylindrical anteriorly, becoming more dorso-ventrally flattened caudally.
Nemerteans possess both a mouth and an anus, connected by a linear gut like that of the turbellarian flatworms, with simple nephridia lined by flame cells. The gut has numerous lateral caeca, which greatly increase the surface area available for digestion.
Nemerteans are recognised as the most primitive organisms to possess a true blood vascular system, consisting of a dorsal vessel and two lateral vessels, interconnected by ‘rungs’. Ribbon worms have no heart, so blood is squeezed through the vascular system by the contraction of musculature. The blood is virtually transparent, but some pigments resembling haemoglobin have been observed. Respiration occurs over the entire body surface.
Habitat & Locomotion
These animals prefer temperate oceans, although freshwater, and even terrestrial groups are also known. Whilst parasitism has not been observed in these animals, some are known to live commensally with other marine invertebrates [Barnes, 1968].
Some ribbon worms live in burrows that have been constructed from slime secreted from their mucus glands. They crawl, leaving a trail of slime similar to that of a gastropod mollusc. Other species use their extended proboscis like an anchor around the substrate, from which they ‘reel’ their bodies in. Alternative forms of locomotion include burrowing and anguilliform swimming.
Other Information
The closest relatives of the nemertea based on similarities in internal and external body construction, are thought to be the less specialised platyhelminthes, which presumably form the ancestral acoelomate plan. The threats to nemertean existence are poorly understood, and their true diversity is surely considerably underestimated. However, deep-sea exploration and dredging activities must surely constitute the greatest threat to their global diversity.
Nemertea Links
http://www.teaching-biomed.man.ac.uk/bs1999/bs146/biodiversity/nemer.htm