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Article by: Muus, Bent J. Danish Institute for Fishery and Marine Research, Charlottenlund, Denmark. Publication year: 2014 DOI: http://dx.doi.org/10.1036/1097-8542.447300 (http://dx.doi.org/10.1036/1097-8542.447300)

Content

Morphology Reproduction Additional Readings

A phylum of worms formerly considered to be a class of the phylum Aschelminthes; commonly called the hairworms, and closely allied to the nematodes. The adults are free-living in aquatic habitats, while the juveniles are parasitic in arthropods. The nematomorphs are found all over the world. They are divided into two classes, the Nectonematoidea and , with a total of 225 species. See also: Nemata (nematoda) (/content/nemata-nematoda/447050)

Morphology

The body is long and slender with a maximum length of 5 ft (1.5 m) and a diameter of 0.02–0.12 in. (0.5–3 mm). The females are longer than the males. The anterior end is rounded with a dark pigmented ring and a terminal mouth. The posterior end may be rounded with a terminal cloaca, or it may form two or three lobes in a forklike structure. The body color is yellowish, brown, or almost black. The body wall consists of three layers: an outer, rather thick fibrous cuticle; an epidermis consisting of a single layer of cells; and innermost, a muscle layer with longitudinal fibers only. The surface of the cuticle may be smooth, or rough with rounded or polygonal thickenings called areoles. These may be flat or may form projecting structures, sometimes with bristles, and they may be perforated by pores and canals. Between the areoles run interareolar furrows, often with wartlike structures and bristles. Special natatory bristles are developed in . The function of the areoles is unknown.

Body cavity

This cavity extends the length of the body. It may be filled with tissue so that only minor spaces are left around the digestive system and the gonads. The digestive tube is always more or less degenerated, and the anterior part is often a solid string of cells. Ingestion of food is impossible, and the intact part of the digestive system seems to be adapted for excretory functions. During the parasitic stage, food is obtained through the body surface by means of digestive enzymes.

Nervous system

This consists of a cerebral mass lying ventrally in the head and a ventral nerve cord which originates in the epidermal layer. Little is known of the sensory organs. Probably the bristles and warts of the cuticle have sensory functions. A rudimentary eye is found in .

Reproduction

The sexes are always separate, and the gonads are paired and stringlike, extending the length of the body. In males, the gonads are connected with the cloaca by sperm ducts. In females, the ovaries form a large number of lateral diverticula in

1 of 3 7/27/2016 10:53 AM Nematomorpha - AccessScience from McGraw-Hill Education https://www.accessscience.com/content/nematomorpha/447300 which the eggs ripen. The oviducts enter the cloaca separately. A sac, which is called the seminal receptacle, extends anteriorly from the cloaca.

During copulation, the male coils itself around the female and places a drop of sperm near her cloacal opening. The sperm cells actively enter the seminal receptable. The eggs are laid in water in strings, and the adults die after egg laying. When hatched, the larvae swim to an aquatic arthropod. They penetrate the body wall of the host by means of their characteristic proboscis, which is armed with hooks and three long stylets. The larvae of some species may secrete a special mucus in which they encyst until they are accidentally ingested by the right host, which may be a terrestrial insect. The development in the host lasts some months without any metamorphosis. When mature, the worms leave the host.

Bent J. Muus

Additional Readings

A. Schmidt-Rhaesa, Handbook of Zoology: Gastrotricha, Cycloneuralia and Gnathifera, Walter de Gruyter, Berlin, Germany, 2013

J. H. Thorp and D. Christopher Rogers, Field Guide to Freshwater Invertebrates of North America, Academic Press, Burlington, MA, 2011

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