Adenophorea: Definition from Answers.com

(Roundworms)

Phylum: Nematoda

Class: Adenophorea

Number of families: 96

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Primarily free-living marine, freshwater, and terrestrial nematodes; considered to be the most primitive form of nematodes

Evolution and systematics

Nematoda, the phylum above the class Adenophorea, have left very few fossil remains. The earliest fossils that contained nematode remnants were found in Eocene strata (the era from about 55–38 million years ago). More authenticated fossils are of nematodes preserved in amber, such as those from fossilized shark muscles and mammals frozen in permafrost. The fossil record is too fragmented to explain much about nematode origins, and conclusions about nematode phylogeny have been mostly based on observations of living species. It is hypothesized that nematodes originated during the Precambrian era, what was the Proterozoic period (about one billion years ago).

As of 1994, about 20,000 species of nematodes have been described, with an estimate by various researchers of the total number of nematode species living on the planet at 80,000–1,000,000. This phylum is considered to have the lowest number of yet-to-be-described species of any animal. Hyman divided nematodes into 17 orders, whereas Chitwood separated them into two main classes, Aphasmidia (now Adenophorea) and Phasmidia (now Secernentea). Controversies still exist, but for the most part, many scientists such as A.R. Maggenti, who helped to develop the classifications, treat nematodes as a separate phylum with two classes, Adenophorea and Secernentea, which were divided based on molecular and morphological characteristics. These two classes are primarily separated (along with other important criteria) with respect to whether they do not possess phasmids (as in Adenophorea) or do possess phasmids (as in Secernentea). Two subclasses are recognized: Enoplia and Chromadoria. In addition, there are 11 orders and approximately 96 families. The total number of species of adenophoreans is estimated at about 12,000 worldwide. Scientific surveys of seabed mud, along with other reliable evidence, suggest that a great number of species are yet-to-be discovered.

Physical characteristics

The majority of adenophoreans are free-living, microbotrophic, and aquatic nematodes. Only a few species are plant parasitic, invertebrate parasites, or vertebrate parasites. They range in size from microscopic to as long as 3.25 ft (1 m) in exceptional cases. Adenophoreans are considered non-segmented pseudocoelomates; that is, creatures possessing a three-tissuelayered body that has a fluid-filled body cavity (pseudocoelom) between the endoderm and the mesoderm (the innermost and middle tissue layers).

A flexible but durable collagenous cuticle protects the body with a series of grooves across the body from head to tail. The non-cellular cuticle, which generally has a smooth surface, can sometimes contain transverse or longitudinal striations and has four layers: endocuticle, epicuticle, exocuticle, and mesocuticle. The cellular hypodermis is the subcuticular layer that secretes the cuticle. Phasmids—which are minute pore-like chemoreceptors that (when present) are usually paired—are generally absent from adenophoreans. Their sensory system contains well-developed amphid apertures, which are postlabial (past the lips) in position, with some species having apertures that are labial (on the lips). The apertures are variable in external shape, being sometimes circular, pocketlike, pore-like, or spiral.

Somatic and cephalic setae, which are elongated structures joined with the cuticle, are common. These tactile sensory organs are usually located around the oral openings. The cephalic sensory organs, which number 16, are setiform to papilloid, and post-labial or labial in position about the head. Deirids, which are paired, porelike organs located in the lateral fields near the nerve ring, are present in some species.

Usually present, hypodermal glands are a thin tissue layer beneath the cuticle that thickens to make the dorsal, lateral, and ventral chords and extend the body's length. In general, they are tactile sensory organs usually located around the oral openings. They contain uninucleate hypodermal cells. A layer of longitudinal muscles underlies the hypodermis.

Bursae (or caudal alae) are rarely found. The ventrally located excretory system, when present, is usually single-celled, usually with non-cuticularized terminal ducts, and lacking collecting tubules. A rectal gland is usually absent. When present, there are three caudal glands located near the posterior region. The muscular esophagus or pharynx (the tube that moves food from the mouth/head to the stomach/intestine) varies in configuration, but the majority of adenophoreans have three esophageal glands: two that are subventral and one that is dorsal. The subventral glands open into the posterior metacarpus. The dorsal gland opens anteriorly into the procorpus or the anterior metacarpus. Its basic structure is corpus (the anterior part is cylindrical) with the basal (bottom) region sometimes swollen in the shape of a bulb. The glands empty their contents into the esophagus to aid in digestion. The tail is the region between the anus and the posterior tip. The male tail is smooth, and lateral cuticular caudal extensions of the tail rarely occur.

Distribution

Adenophoreans are found worldwide, especially in water-filled areas around vegetation and in soils, on plants, and in animals.

Habitat

Adenophoreans are commonly found in most kinds of habitat (except for desert regions) throughout the world, particularly in marine sediments. They inhabit a wide variety of habitats, including both water and soil.

Behavior

Their behavior is classified as being usually free living, but with some species behaving as parasites.

Feeding ecology and diet

The free-living species are predators, carnivores, herbivores, and omnivores, feeding mostly on bacteria, fungi, and soil organisms. The parasitic species feed on nutrients (such as blood, body fluid, intestinal contents, mucus) found within their hosts. Adenophorean worms generally have some form of stylet, a hard, sharp spear, which is used for feeding. Muscles move the stylet in and out, allowing the worms to puncture cells. Once opened, the worm will empty the contents of the cell.

Reproductive biology

Male adenophoreans sometimes have thin cuticle extensions on both sides of the anus called bursae, which are fluid-filled body sacs. When present, bursae are used to hold females during copulation. Male testes, when present, are two in number and are shaped similar to the female ovary. The spicules, the male copulatory organ, are paired. Males have a single, ventral series of papilloid or tabloid pre-anal supplements. Sperm accumulate in the seminal vesicle and exit through the anus. During mating, the rigid spicules insert into the vagina and form a passageway for the sperm. The female ovary contains germ cells that produce eggs. Fertilization, mostly initiated by male sperm, takes place in the uterus, with eggs released through the vagina. (Most species produce males and females, but some species only produce hermaphrodites, in which both male and female structures are contained in the same individual.) Most eggs are about the same size and shape.

Conservation status

No adenophoreans are listed on the 2002 IUCN Red List. They are found worldwide and frequently occur in very great densities.

Significance to humans

Living free and as parasites of animals, insects, and plants, adenophoreans constitute an important part of nature and to the activities of humans. As crops are cultivated to feed the world's population, adenophoreans become more numerous as they feed on agricultural plants. Parasitic adenophoreans cause yield losses by themselves or they may join with other organisms such as viruses, fungi, and bacteria to advance disease development in plants. They also cause loss of nutrients and water in the plant, thus increasing the plant's susceptibility to other dangers. Adenophoreans, when infecting a human, can cause various diseases, and in some circumstances, death to the human host. On the other hand, studies have shown that the appearance of adenophoreans is a good indicator of biodiversity, which is important to the health and survival of humans. Adenophoreans help to cycle carbon and nitrogen and to breakdown organic matter in the soil environment.

Species accounts

Desmoscolex squamosus
Greeffiella minutum
Nygolaimus parvus
Mermis nigrescens
Trichina worm
Human whipworm

Resources

Books:

Farrand, John, ed. The Audubon Society Encyclopedia of Animal Life. New York: Clarkson N. Potter, 1982.

Bird, Alan F. The Structure of Nematodes. New York: Academic Press, 1971.

Chitwood, B. G., and M. B. Chitwood. Introduction to Nematology. Baltimore: University Park Press, 1950.

Croll, Neil A., and Bernard E. Matthews. Biology of Nematodes. Glasgow and London: Blackie and Son Limited, 1977.

Levin, Simon Asher, ed. Encyclopedia of Biodiversity. San Diego: Academic Press, 2001.

Maggenti, Armand. General Nematology. New York: Springer-Verlag, 1981.

Malakhov, V. V. (translated by George V. Bentz, edited by W. Duane Hope). Nematodes: Structure, Development, Classification, and Phylogeny. Washington, DC, and London, U.K.: Smithsonian Institution Press, 1994.

Mehlhorn, Heinz, ed. Encyclopedic Reference of Parasitology: Diseases, Treatment, Therapy, 2nd ed. New York: Springer, 2001.

The New Larousse Encyclopedia of Animal Life. New York: Bonanza Books, 1981.

Parker, Sybil P., ed. Synopsis and Classification of Living Organisms. New York: McGraw-Hill Book Company, 1982.

Poinar, George O. Jr. The Natural History of Nematodes. Englewood Cliffs, NJ: Prentice-Hall, Inc., 1983.

Stone, A. R., H. M. Platt, and L. F. Khalil, eds. Concepts in Nematode Systematics, Special Volume No. 22. London: Academic Press, 1983.

Wharton, David A. A Functional Biology of Nematodes. London: Croom Helm, 1986.

[Article by: William Arthur Atkins]

Adenophorea

Pratylenchus, an Adenophorea species in the Tylenchida order
Scientific classification
Kingdom:	Animalia
Phylum:	Nematoda
Class:	Adenophorea
Subclasses
Enoplia Chromadoria

	Aphasmidea (invertebrate zoology)
	Chromadoria (invertebrate zoology)
	Enoplia (invertebrate zoology)

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