Nemertea

Introduction

How would you like to tie a ribbon in your hair? Nemerteans are called ribbon worms because of their long unsegmented bodies. Be careful when you handle these worms; their proboscis catches prey by being very sticky.

Morphology

Nemerteans are 10 to 40 mm in length and usually a pinkish color. A feature unique to this phylum is their eversible proboscis that lies dorsal to the digestive tract and is contained within a hydrostatic chamber called the rhynocoel. The proboscis is a blind-ended tube that often has nail-shaped stylets attached to the end that aid in capturing prey. It may be everted and retracted using hydrostatic pressure, or it may have retractor muscles to pull it back into the body. Nemerteans have a ciliated epidermis covering several muscle layers.

Juvenile nemerteans can swim, but adults are only able to crawl by combining muscular movement and the action of epidermal cilia. Their quickest mode of locomotion occurs when they evert their proboscis, attach it to the substrate, and use it to pull their body forward. Nemerteans secrete copious amounts of mucus to aid in locomotion.

Their sensory system consists of a fairly complex cerebral ganglion and longitudinal nerve cords. Eyes are present in freshwater forms.

Metabolism

Because they are so small, gas exchange takes place across the body surface.

Osmoregulation is under the control of the cerebral ganglion and involves the circulatory system and protonephridia, but the mechanism is not well-studied. The secretion of copious amounts of mucous helps decrease the influx of freshwater.

Nemerteans have a closed circulatory system that contains a colourless fluid. This fluid may have pigmented granules, some of which contain hemoglobin, while the function of others is unknown.

Nemerteans have a protonephridial excretory system.

Reproduction

Freshwater nemerteans are hermaphroditic; most are sequential hermaphrodites, developing male gonads first. They only reproduce sexually, although self-fertilization is possible. Fertilization is external. Many gonads are present in the folds of the intestine. Nemerteans do not possess a distinctive larval form.

Ecology

Most nemerteans are marine, but twelve species occur in freshwater. These species can be found in the benthos of lakes and streams; one genus (Prostoma) often occurs among filamentous algae in lakes.

Nemerteans are benthic predators that capture their prey with their sticky proboscis and swallow it whole. They feed on crustaceans, nematodes, turbellarians, insect larvae and polychaetes. They are most active at night.

Nemerteans do not make up a large portion of the diet of any predator. They secrete large amounts of mucus as a defense mechanism.

Idiosyncratic Inverts

Nemerteans release copious amounts of mucus that acts to deter predators, aids in locomotion, and decreases the influx of freshwater.

Glossary

Citation

Hebert, P., & Ontario, B. (2007). Nemertea. Retrieved from http://www.eoearth.org/view/article/154810

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