Nemertini


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Related to Nemertini: phylum Nemertea, Nemertean worm

Nemertini

 

(also Nemertea), a phylum of invertebrates, sometimes regarded as a subphylum of the Scolecida.

The body is bilaterally symmetric, nonsegmented, and usually very elongated and without appendages. At the anterior end is the opening for the proboscis and the oral opening; the anus is at the posterior end. Lineus longissimus, the largest nemertine, has a body length of 30 m with a diameter of 1 cm. Most nemertines are no more than 20 cm long and are 1–5 mm wide. The coloration is varied and is sometimes arranged in a mottled pattern. The body is covered with a single layer of ciliated epithelium that overlies the musculocutaneous sac. The spaces between the internal organs are filled with areolar tissue. The long muscular proboscis, which can be everted as well as coiled up within the animal’s body, is used for defense and attack. The resting proboscis is drawn into a special sheath that lies above the gut. In some nemertines, the proboscis is equipped with one or more stylets.

Nemertines have distinct digestive, circulatory, excretory, and nervous systems, as well as tactile organs, chemoreceptors, eyes, and organs of equilibrium called statocysts. They are dioecious and have numerous sacciform sex glands. Males and females are practically indistinguishable by outward appearances. Nemertine embryos can develop directly or can pass through several stages of transformation. Certain nemertines develop a characteristic larva called the pilidium.

There are more than 110 genera of nemertines with more than 1,000 species; more than 100 species are found in the USSR. Most nemertines are free-living predators distributed throughout all the oceans and seas; several species are parasites or symbionts of mollusks, crabs, and starfish. About 30 species are found in fresh water and more than ten species live on land. Nemertines move by contracting the musculocutaneous sac; the smaller forms move by beating the cilia. Nemertines live mostly on the sand; sometimes they burrow into the sand or maintain a pelagic mode of existence. Many nemertine species serve as food for fish.

REFERENCES

Dogel’, V. A. Zoologiia bespozvonochnykh, 5th ed. Moscow, 1959.
Zhizn’ zhivotnykh, vol. 1. Moscow, 1968.

V. S. KOROTKEVICH