Secernentea: Definition from Answers.com

(Secernenteans)

Phylum: Nematoda

Class: Secernentea

Number of families: 60+

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Almost exclusively terrestrial nematodes, generally parasitic of plants and invertebrate and vertebrate animals; they are bilaterally symmetrical and non-segmented

Evolution and systematics

Nematoda, the phylum above the class Secernentea, has left very few fossil remains. The earliest fossils that contained nematode structures were found in Eocene strata (the era from about 55–38 million years ago). More authenticated fossils are of nematodes preserved in amber within such items as fossilized shark muscles and mammals frozen in permafrost. The fossil record is too fragmented to explain much about nematode origins, so most conclusions about phylogeny are 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).

Earlier in the classification process Chitwood separated hematodes into two main classes, the Phasmidia (now Secernentea) and Aphasmidia (now Adenophorea). Controversies still exist, but for the most part, scientists, such as A. R. Maggenti, who helped to develop the classifications under this system, treat nematodes as a separate phylum with two classes, Adenophorea and Secernentea, which have been divided based on molecular and morphological characteristics. These two classes are primarily separated (along with other important criteria) with respect to whether they possess phasmids (as in Secernentea) or do not possess phasmids (as in Adenophorea). The total number of species of secernenteans is estimated at about 8,000 worldwide, with scientific surveys suggesting that an enormous number of species has yet-to-be discovered. There are six orders and the number of families ranges from 60 to 89.

Physical characteristics

Secernenteans vary greatly in size from microscopic to several feet long. The largest known secernentean, which is up to 30 ft (9 m) in length, lives in the placentas of female sperm whales. The body of secernenteans consists of a flexible cylinder that tapers at both ends, with a pointed tail and a blunt head. They are considered non-segmented pseudocoelomates; that is, creatures possessing a three-layered body that has a fluid-filled body cavity (pseudocoelom) between the endoderm and the mesoderm (the innermost and middle tissue layers).

A flexible but tough collagenous cuticle surrounds the body with a system of grooves across the body from head to tail, which protects them internally. The non-cellular cuticle varies from four to two layers and is almost always transversely striated. Laterally for most of the body length, the cuticle is generally modified into a wing area that is marked by longitudinal ridges; generally, this region is only slightly above the normal body contour. However, in some parasitic forms, it may extend out a distance equal to the body's diameter. The cellular hypodermis is the subcuticular layer that secretes the cuticle.

The sensory system contains phasmids, which are a pair of bilateral cuticular, glandular organs, situated laterally in the caudal (posterior to the anus) region and opening to the surface by a slit or pore. Also known as precaudal glands, phasmids are unique to the secernenteans, in which their function is believed to be sensory. At the other end are pore-like amphid apertures, which are a pair of glandular chemosensory organs situated dorso-laterally in the cephalic (head or anterior) region and opening through the cuticle. Although usually pore-like, in isolated instances the aperture can be an oval or a cleft. The apertures show little variation throughout the secernenteans. The amphids are always labial (located on the lips). The external amphidial aperture is usually less well developed than in adenophorean worms.

Somatic and cephalic setae, which are elongated structures jointed with the cuticle, are rare. When present, the cephalic sensilla are located on the labial region, and they are pore-like or papilliform. In males, there may be caudal setae. In females, somatic setae are absent. Generally, sixteen sensilla are present in the shape of two circles (an inner circle of six, and an outer circle of 10). In some parasitic groups, the number of cephalic sensilla may be reduced. Deirids, pairs of pore-like sensilla that usually protrude above the surface of the cuticle, are usually present on the cervical region near the level of the nerve ring.

Secernenteans contain no hypodermal glands, but the hypodermal cells of the hypodermis (a thin tissue layer beneath the cuticle that thickens to form the dorsal, ventral, and two lateral hypodermal chords, and extends the length of the body) are usually multinucleate (syncytial: more than two nuclei), but may also be uninucleate (cellular: one nuclei). These divide the muscle cells into four fields. A layer of longitudinal muscles underlies the hypodermis.

Bursae, or caudal alae, are sublateral projections generated by a longitudinal widening of the cuticle. It is common for them to be present within male secernenteans, each looking like a fluid-filled body sac. The thin cuticle extensions are located on both sides of the anus, specifically on either side of (or sometimes surrounding) the cloaca (the urogenital opening) of males. The well-developed excretory system is in the shape of an H or U. It is a simple tubular system that is located in one or both of the lateral hypodermal chords, and embedded between the three cell bodies in the hypodermal chord. The system opens ventromedially by way of a cuticularized duct. The rectal part of the gland system is usually present. They have no caudal glands.

The muscular esophagus or pharynx varies in configuration, but the majority of secernenteans 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 back tip of the body.

Distribution

Secernenteans are distributed worldwide in terrestrial environments, and only rarely in regions containing water habitats.

Habitat

Secernenteans show considerable diversity in their habitats, including free-living microbotrophs, plant parasites, invertebrate parasites, and vertebrate parasites. They are rarely found in marine and freshwater habitats, being almost exclusively terrestrial. They are mostly parasites, living on or within their plant or animal hosts. When free living, they often live in soil.

Behavior

This class encompasses most of the plant, invertebrate, and vertebrate parasites in the Nematoda phylum. Within the class, there are more than 3,000 animal parasites and 2,000 plant parasites. Their lifestyle is classified primarily as parasitic, but there are some free-living species. When parasitic or a combination of parasitic and free-living, secernentean behavior primarily revolves around their hosts. Each species has developed particular ways to infect their hosts, adapting their behaviors to best suit their needs while living off their hosts.

Feeding ecology and diet

They feed mostly on bacteria, fungi, and soil organisms, as well as on the nutrients such as blood, body fluid, intestinal contents, and mucus found within their hosts. Secernentean worms generally have some form of stylet, a hard, sharp spear that is used for feeding. Muscles move the stylet in and out, allowing them to puncture cells. Once opened, the worm will empty the contents of the cell.

Reproductive biology

The female ovary contains germ cells that give rise to eggs. Fertilization, most of the time by male sperm, takes place in the uterus and eggs are released through the vagina. Males have one testis (monarchic), when testes are present. Most species produce males and females (that is, dioecious), but some species only produce females, in which both male and female structures are contained in the same individual. In those few cases the species are hermaphroditic. Most eggs are about the same size and shape. Males produce sperm in the testes, which are shaped similar to female ovaries. Sperm accumulate in the seminal vesicle and exit through the anus. Males have accessory copulatory organs, called spicules. During mating, a spicule becomes rigid so it can be inserted into the vagina in order to form a passageway for the sperm. Male bursae, usually numbering four, are used to clasp the female during copulation. Males possess paired preanal supplement glands (or genital papillae) in two sublateral rows on the ventral side of the body. The glands are used for secretion and attachment, and function during copulation.

The life cycle is generally a straight cycle going from fertilized egg, through four juvenile (often called larval) stages involving a set number of cell divisions, and into adulthood. Secernenteans generally produce offspring through internal sexual reproduction. Fertilization occurs in the female's uterus, where the zygote is then placed inside a tough shell. Development from egg to adult is generally similar for all species, although many differences exist to the norm (often times brought about by environmental conditions and special types of life, as two examples). Usually a young juvenile hatches from the egg. It usually resembles the adult except in size and in the maturity of its sex organs. Each of the four larval stages (usually referred to as L1, L2, L3, and L4) is separated from one another by the complete shedding of the outer layer (what is called ecdysis, or molting in other animals), including the lining of the mouth and rectum. There is no increase in the number of cells after hatching, with growth coming exclusively from an increase in the size of the original cells. Development in successive stages is gradual over all, however organs themselves can develop at rapid rates.

Conservation status

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

Significance to humans

As crops are cultivated to feed the world's population, secernenteans become more numerous as they feed on agricultural plants. Parasitic secernenteans cause yield losses and they may join with other soil-living organisms such as bacteria, fungi, and viruses to advance disease development in plants. They also can cause loss of nutrients and water into the plant, thus increasing the plant's susceptibility to other dangers. Secernenteans, when infecting humans, can cause various diseases and, in some circumstances, death to the human host. When infected animals are used as food or kept as pets for humans, these parasites can cause and transmit various diseases and, often, death to the hosts. On the other hand, free-living forms of secernenteans can be good indication of biodiversity, and important to the health and survival of humans. They help to cycle carbon and nitrogen and to breakdown organic matter in the soil environment.

Species accounts

Maw-worm
Threadworm
Canine heartworm
African river blindness nematode
Dog hookworm
Rat lungworm
Barber's pole worm
Citrus spine nematode
Cod worm

Resources

Books:

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.

Mehlhorn, Heinz, ed. Encyclopedic Reference of Parasitology: Diseases, Treatment, Therapy. 2nd edition. New York: Springer, 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: Smithsonian Institution Press, 1994.

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]

Secernentea

Systematics

Footnotes

References

	Phasmidea (invertebrate zoology)
	Phasmidia (invertebrate zoology)
	Rhabditia (invertebrate zoology)

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