A Spineless Column by Ronald L. Shimek, Ph.D.


"If all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable... we would find its mountains, hills, valleys, rivers, lakes and oceans represented by a film of nematodes."

Nathan Augustus Cobb, 1914

Worms' Worms

Given the immense variety of animals, it might appear difficult to pick a single type of animal as being characteristic of all the Earth's animals. The diversity of animal life is not, however, distributed equally across all the various "types" of animals, and some types include many more species than others. These species-rich animal groups surely are more "characteristic" of all animals than are those groups containing only a few different varieties. Each distinctive "type" of animal life may be considered to be the taxonomic groups called "phyla" (singular = phylum). A phylum is an arbitrary grouping of similar animals thought to represent a unique and specific "body plan" or architecture. Most taxonomists consider between thirty and fifty named phyla as constituting the complete array of animals' body plans. These represent the gamut of animals' structural patterns, from the simplicity seen in a sponge or coral polyp to the profound structural complexity seen in a blue whale or a butterfly's caterpillar.

In some ways it is easy to array all of these various body plans in some sort of order, from simplest to most complex, for example, and compare them. Because such comparisons are so easy, this process has been hammered to death by zoologists for the last couple of centuries, with the objective of trying to find some sort of order in life's immense diversity. Visually examining and comparing the body plans results in… chaos, confusion, argument and resignation. There is no clear way to simply look at, examine and arrange the various phyla in an inherently obvious and rational arrangement. Instead, there are a LOT of "inherently obvious and rational arrangements," some of which are more "inherently obvious" than others. And, all of these various arrangements vary between being somewhat different to radically different from each other, and all of these constructed and artificial patterns depend upon assumptions made during the comparisons and orderings. Assumptions of this nature are always wonderful things, as they are based on those delicious and delicate ambiguities known in the vulgarity of college life as "weasel words," and in the morass of national politics as "opportunity." As an example, while it is easy to conceptualize the concept of "complexity" with regard to animals, it is well nigh impossible to erect some precise, unambiguous and universally agreed upon "measure" or standard of complexity. Visually comparing two very different, yet complex, body plans, such as the one for arthropods and the one for fishes, is very difficult, as they share almost no structural similarities. Both designs lead to very complicated animals, but in very different ways. There is really no way to compare such "finned" apples and "clawed" oranges to come up with a relative index of complexity. Problems such as this caused stagnation of research into the relationships between the various animal groups; from about 1940 until the late 1990's few new ideas, and almost no new evidence, were presented about how animal groups were interrelated. There was a lot of passionate discussion about their relationships, generally accompanied by some serious waving of arms, but as nobody had any unambiguous data, independent of conjecture, there was no way to resolve the various competing hypotheses. All of this changed with the development of genetic comparisons, whereby various components of the actual genetic material that determined the structures could be compared. This allowed for the determination of which animal groups had the most similar genetic material or, in other words, which groups were most closely related. Applied within a wide series of group-by-group comparisons, this process has facilitated the development of a framework of relationships that is based on both genetic and structural characteristics relating most major animal groups to one another. Nonetheless, even with this group of methodologies, there are still some unanswered questions, both of technique and result.

Such a framework of relationships shows which animal types share lines of descent from common ancestors. Applying some biological acumen to the changes that occur when lineages diverge can, however, also lead to an appreciation of the ways in which animals have come to exploit different environments. Generally, the more complicated the animal's basic body plan, the more different types of habitats in which the animals sharing that plan can exist. It is generally conceded that arthropods, chordates and mollusks share a level of complexity not reached by other animals. Consequently, and not surprisingly, arthropods, mollusks and chordates are found in just about every habitat on Earth. Animals with a simpler level of organization, such as sponges or corals, are much more restricted in the habitats they can occupy. As a result, we might suppose that the animal considered to be most characteristic of all animal life would be either an arthropod, a mollusk or a chordate. Perhaps, but there is another option.

The worms in the Phylum Nematoda constitute the one notable exception to this relationship between structural complexity and diversity of habitats occupied. These worms, commonly called "round worms," are fundamentally simple in structure and, yet, are not only found almost everywhere, but also are found in just about every other type of multicellular living organism. It is quite possible that there are more different species of nematodes than of all other animals combined. Because of their widespread existence, it might reasonably be said that the most characteristic and ubiquitous form of animal life is a nematode. The nematodes' body plan is truly the design of a worm's worm.

Figure 1. An unidentified free-living nematode found in marine sediments. Its basic structural simplicity is evident.


Given that nematodes live everywhere and in everything, it is reasonable to ask, "Why?" What special properties or characteristics that nematodes alone possess have allowed this degree of diversity? When a group of organisms has diversified into many different forms, this is referred to as an "adaptive radiation." The organisms' group appears to have spread from one central area, or diversified from one ancestral type, much as light radiates from a single source. In the process, as they have encountered new habitats or situations, the organisms have adapted to them.

Figure 2. The process of adaptive radiation needs populations of organisms spread over a wide variety of conditions. New species typically arise when organisms living under suboptimal or marginal conditions undergo genetic changes that allow them to exploit those conditions more fully than their ancestors.

Diversification of animals is not an automatic process, and a good many groups have never diversified very much. Typically, diversification requires some properties of form and function that allow exploitation of new habitats or life-styles. One of the reasons that there are so many species of some types of animals such as insects, snails or fishes is that their body's form is "plastic" or "malleable" when subjected to evolutionarily selective pressures, and it can change or form into new shapes that are adapted to particular habitats or lifestyles. Such "variable" organism groups may also have more genetic material that may be modifiable, which in the end, may result in structural variations.

That type of diversification appears to be exactly the opposite of what has occurred in the nematodes. The great diversity seen in the nematodes notwithstanding, the nematodes are a VERY homogeneous group with regard to morphology. Unlike many of the more complicated types of animals, the key to nematodes' diversity appears, in many ways, to be their structural simplicity, coupled with a design that allows the exploitation of an extremely common environment. The nematode body "design" appears to have been "preadapted" to the exploitation of many environments.

The Worm's Outside Surface

The nematode's body is a thin cylinder which tapers to a fine point at either end. The common name "round worm" refers to these worms' cross-sectional appearance, which stands in contrast to the many "flatworms" seen so commonly in marine aquaria and elsewhere. Their lack of any indication of true segmentation separates and distinguishes them, as well, from the annelid worms, such as "bristle" and earth worms.

The outermost covering of a nematode's body is a non-living proteinaceous cuticle, and although some species have a superficially annulated cuticle, they lack the internal divisions characteristic of truly segmented animals. Nematodes also lack any appendages, although they may possess sensory bristles or hairs. The nematodes' cuticle is a complex and multilayered structure that is secreted by an underlying epidermis. The cuticle's major layers consist of the protein collagen, which also is the major constituent of vertebrate ligaments. Collagen is NOT elastic and NOT stretchable. Generally, at least three layers of collagen are found secreted tightly adjacent to one another in an arrangement not unlike the layers of wood in plywood or some other laminate. The "grain" of the various layers alternates in direction, giving strength and resiliency to the body wall.

Visible external structures are relatively few. Nematodes have a few complex sensory organs, but eyes are rare. Occasionally, when eyes are present, they are found inside the throat. Other sensory structures are located on various bumps and pits in the cuticular surface. Most of these structures are innervated and are presumed to be sensory, but the exact stimuli that they respond to are unknown. Most researchers presume that they possess both tactile and chemosensory capabilities. Nematodes may also possess "sensory" pits called "amphids" at either side of their body just at the back of their head, but their sensory modality is unknown. Depending on the species, there may be additional papillae in the "cervical region." Additionally, some nematodes have structures called "phasmids" which appear to be adhesive organs. Phasmids may be duo-gland adhesive structures. As the name "duo-gland" implies, these structures consist of two glands working together. When the animal wants to remain attached to something, one gland secretes an adhesive, and when it wants to move, the other secretes a releaser substance that dissolves the adhesive and the worm will be on its way.

It is worth noting that some of these descriptive terms are rather ludicrous. Nematodes have neither a defined head nor a neck; however, both terms are still applied to them. Most free-living nematodes are transparent, with internal divisions that do delineate something that could be called an anterior region containing the mouth's parts; that part of the animal is referred to as the head. The area just behind this head is, of course, the neck or cervical region.

The arrangements of exterior structures such as the papillae, and the presence or absence of amphids and phasmids, constitute some of the major taxonomic characters used to discriminate the phylum's various subdivisions. To identify species or other taxonomic subdivisions requires examination of these structures, and they may be VERY hard to see. Most folks just give up at this stage if they are trying to identify the worms, and just call 'em all "nematodes."

Figure 3. This is a diagrammatic representation of the anatomy of a typical nematode; left: female, right: male.

Figure 4. Diagrammatic representation of the cross-section of a typical nematode, showing the relationships of the body's major structures, with the exception of the gonads.

Muscles and a Liquid Skeleton

Underlying the cuticle is the epidermis, sometimes called the "hypodermis," a layer that marks the animals' actual living surface. Inside the epidermis, most of the remaining body wall structure is composed of muscles. No muscles run around the body of any nematode; their only muscles run the length of the worm. Much of the worm's structure is dictated by this musculature. Any muscle can only actively contract. Once contracted, all muscles must be actively stretched back out to their expanded length to be able to contract again. In animals such as ourselves or crabs, this expansion is done by that system of levers we call our skeletons. In animals with a lever-action skeleton, each muscle is paired; for each muscle that contracts, another muscle must, by its contraction, force the lever to pull the first muscle back to full extension. A good example of muscle pairing is the biceps and triceps pairing in humans' upper arm. The biceps contract to flex the elbow, and stretch the triceps, and vice versa.

To stretch their muscles back to full extension, nematodes do not use levers, but instead use water enclosed in a bag and held under high pressure. Such an enclosed water volume used to antagonize muscles is called a "hydrostatic skeleton," and nematodes probably have the most well-developed, simple hydrostatic skeleton in the animal kingdom. The fluid is enclosed in the animals' body cavities and contained there by the strength of the body wall. This is an extremely high pressure system; in large worms, the internal pressure can exceed 250 mm Hg (more than normal human blood pressure).

The hydrostatic skeleton is maintained by two major factors. First, the complex and non-expandable, non-elastic cuticle acts to contain the body and keep it from expanding. The second factor that is necessary for this system's functionality is the physical process of osmosis. Osmosis is simply the diffusion of water across a membrane in response to a water concentration gradient. Because the worm is filled with fluid comprised mostly of water, but also containing various chemicals, water is less concentrated inside the cell than outside it. Hence, water tends to ooze into the worm's body cavity. The collagen layer and osmotic pressure tend to provide pressure to "fully inflate." Fluid enters the worm by osmosis, and is held back only by the physical properties of the cuticle. If the cuticle were not present, and the worms were otherwise the same as normal, they would explode from the influx of water.

The muscles in nematodes are arranged into four groups, one for each quadrant of the animal. If the nematode is visualized in cross-section, it will be seen that right and left dorsal and ventral muscle groups run the length of its body. These dorsal and ventral muscle groups are enervated separately and are contracted separately in an alternating sequence to produce the characteristic dorso-ventral flexion that constitutes the ONLY motion available with the pattern of muscles found in these animals. This whip-like up-and-down motion, with the animal flexing first one way and then the other, is characteristic of nematodes. Nematodes can't twist and turn because they have only longitudinal muscles, and they can't move by alternately contracting and expanding various sections of their body's musculature. They can perform only an all-or-nothing dorsal or ventral flexion.

This system is functionally very simple. When a muscle group contracts, it tends to shorten the body on the top or the bottom. This, in turn, tends to reduce the body's volume, but as the fluid filling the body is incompressible, the volume must remain the same. Consequently, internal pressure increases, the opposite muscle band extends, and the worm bends. The antagonists of the contracted muscles are the muscles on the animal's opposite half, and the force is transmitted by the hydrostatic skeleton maintained and coupled with the cuticular fibrillar arrangement.

To reiterate, the only motion that nematodes can do is to flex, either in the up (dorsal) or down (ventral) direction. They simply cannot move in any other manner. Such motion is inefficient for movement unless the animal is in a viscous medium (such as a sand bed, soil, tissue or bodily fluids) or fastened to substrate. In such media, however, this motion is quite efficient, and people who have some large nematodes (Ascaris) living in their guts often can feel them actively swimming from place to place. The efficiency of such movement in viscous media is also the answer to the puzzle of why there are there so many kinds of nematodes. The answer is simply that these animals are superbly designed to move in viscous substrates, and these substrates are everywhere, from all aspects of the soft-sediment marine environment to terrestrial soils to the tissues found in all animals and plants.


As befits an animal with such a simple set of locomotory motions, the nervous system is simple. It consists of a nerve ring around the foregut with a ventral swelling called the brain. Nematodes also have four nerves running the length of their body: a dorsal nerve, a ventral nerve and a lateral nerve at the midpoint on each side. If you examine a worm cut perpendicularly to its long axis (termed a cross-section), you would see that the nerves divide the animal into equal quadrants. The lateral nerves are largely sensory, and the dorsal and ventral nerves are largely motor nerves conducting impulses that cause muscular contraction. So, the lateral nerves conduct impulses to the brain, while the dorsal and ventral nerves conduct impulses from the brain to the muscles. Located ventrally on the anterior nerve ring, the brain is a swelling of nerve tissue referred to as a ganglion. It is doubled at its anterior end. There is an additional aggregation of nerves, or ganglion, at the posterior end called the anal ganglion.

Simple Innards

Nematodes have a simple gut. Their mouth often has biting jaws, and in parasitic forms they may actively bite their way through tissue. The free-living forms use their jaws to catch prey or to grasp sediment that is eaten. The remainder of the gut is a simple tube, only one cellular layer thick, without folds, loops, pouches or membranes connecting it to the body wall. It runs from the mouth-throat region at the animal's anterior end to the rectum at its posterior end.

The high hydrostatic pressure presents problems for the animal; simply put, this pressure tends to force contents out of the gut. Consequently, a rather complex system of valves controls nutrient passage. Food has to be actively pumped into the gut, and the throat region, called the pharynx, is basically a muscular pump designed to do just that. Defecation occurs by the relaxation of some of the musculature near the anus. High internal pressure then expels the feces. Due to the high internal hydrostatic pressure in large worms, defecation can be an impressive event; a large pig or human round worm of the genus Ascaris can spray its feces about 10 feet vertically into the air or about 30 feet laterally! These animals normally live in the host's small intestine, and in these situations, the feces travel only a short distance from the animal. Other than the muscular valves around either end, the gut is simple, consisting of just a single digestive epithelial layer, with no muscles lining it. Both extra- and intra-cellular digestion occur, and nutrients travel from the gut to the body wall in the fluid of the body cavity. Nematodes secrete ammonia as their primary waste product. Most nitrogenous wastes exit through their gut.

Figure 5. The anterior end of a small marine nematode showing the muscular pharynx (throat region) necessary to pump food into the gut.

Reproduction and Development

The sexes of nematodes are separate and visually distinguishable. The male worms have a cloaca, a common opening for the anus and the male genital pore. This is located slightly in front of the worm's posterior end. The male generally has a pair of copulatory spicules which protrude from his cloaca. These assist in holding the mate's genital aperture open during copulation. The female genital aperture is located about midway along the animal, on her ventral surface. As far as I know there are no hermaphroditic nematodes. All fertilization is internal, and the male often has accessory structures for pumping the sperm in against the female's high internal pressure. Cilia don't work under high pressures, and the sperm are amoeboid.

The embryological development is extremely determinate; this means each cell's fate is determined prior to fertilization. After each cell division, the chromosomes undergo diminution - they get broken and reduced except in germ cell lineage. Once development is complete, cell division ceases, except in the gonads. No bodily, or somatic, cell can be stimulated to divide again, and this means the animal has no way of repairing injury or damage. Nematodes are "eutelic;" this means that EVERY individual of a given species has EXACTLY the same number of cell nuclei in EXACTLY the same position in its body. For example, all individuals of the nematode version of a "lab rat," Caenorhabditis elegans, have 959 somatic nuclei. Interestingly enough, while they may have "cell" nuclei, what they lack are actual cells. Most bodily structures in nematodes are syncytial; the nuclei that normally would be expected to exist within cells are found in masses of protoplasm, but no cell walls delineate actual cells within these masses. Lacking cells and cellular division, nematodes are incapable of repairing injury and if damaged, they will die.

As in the arthropods, the cuticle constrains growth, so to increase in size, the animals must molt. Most nematode species have five life history stages (resulting from four molts). Growth is achieved primarily by an increase in cellular size, and the animal's size is NOT proportional to molt stages. Many species remain very small until their last molt and then increase significantly in size. Round worms are taxonomically placed into the Phylum Nematoda, in the branch of the animal kingdom called the Ecdysozoa, as they must molt to grow. The term Ecdysozoa is derived from roots meaning animals that "strip" or molt, hence all animals that molt are put into this group. While Cobb in his quote above was undoubtedly not thinking of marine reef aquaria, the quote that started this article is just as undoubtedly applicable to them. Nobody really has an honest clue as to the absolute diversity of nematodes; their basic structural similarity makes distinguishing species an absolute nightmare. Nonetheless, what appear to be reasonable estimates of the number of species range upward to over 10,000,000.

An Identification Nightmare

The apparent consistency of form exhibited in the group is so overwhelming that nonspecialists often say that "There is only one nematode species; it just comes in different sizes." This is not at all true, (2), but the differences between the species often appear to be exceptionally subtle, and it takes thorough training to be able to distinguish many species. Some of this may be due to our problems in defining species by the appropriate criteria. Although almost all biologists give lip service to the "biological" species description of Ernst Mayr which states that "a species is an interbreeding unit," in practice, most biologists fall back onto the "morphological" species description.

Basically, the "morphospecies" concept implies that if there are consistent and significant differences between individuals from two populations, then these two populations are two species. Or... phrased another way, "If I can tell 'em apart, they come from two species; if I can't tell 'em apart, then they form one species." There are some obvious problems with this approach, not the least of which is that it totally ignores the animals' actual biology. Additionally, it depends upon a subjective "feeling" of difference. In other words, how far apart do two animals have to be to be in different species, and just exactly how do taxonomists measure that magnitude of difference?

Keeping these little problems in mind, you have to realize that we know very little about the natural history of any free-living nematode, and that coral reef dwelling forms are particularly poorly understood and very difficult to tell apart. Because of this we have to fall back on the morphological species concept. We can't use the biological species concept simply because we don't know enough about their biology. Nematologists, fortunately, seem to be a rather conservative crowd, and seem to require a lot of differences between their animals before they refer to them as two different species. That magnitude of differences should help us identify the beasts, but this is not really the case. That is because these differences are found in rather obscure organs and external features, such as amphids and phasmids, that are often difficult for the non-specialist to observe or appreciate. Nonetheless, this approach tends to minimize the number of new species described. It may also severely undercount species if those species are separated by different criteria.

On the other hand, perhaps the most discerning of nematologists, the worms themselves, are not bound by such obvious cues and may discern much more subtle differences. If this is true, there may be dozens of species within what appears as a single "morphological" species. This is a situation that occurs in many phyla of marine animals, and it should be expected in nematodes. Knowledge of various species' biological interactions could help to distinguish such cryptic species. Unfortunately, this knowledge is lacking.

Nematodes occur everywhere and are among the most ecologically important animals in EVERY ecosystem - including ourselves and our captive reef ecosystems. Some are free-living, but as with the flatworms, the vast majority of species are parasitic. The free-living forms are generally pretty small, ranging in size from 1 mm to about 20 mm (0.04 to 0.8 inches); however, they are often exceptionally abundant. In many terrestrial areas they range in numbers upwards of several billion animals per acre. Although many parasites are small, most are between 0.5 mm and 60 cm (0.2 to 24 inches) in length, but the largest nematode is in the species Placentonema, which is parasitic on the placentas of sperm whales, and reaches lengths of about 9 m (30 feet). Interestingly, and probably because it is so large (and difficult to study; feature, if you will, the problems of finding an animal that lives in the placentas of sperm whales. Step number 1, find a pregnant sperm whale. Step number 2…), this species has become the substance of fable; it is hard to find any consistent information about it. For example, its diameter is variously listed in different references as being anywhere from 0.03 to 2.5 cm (from 1/8th to about 1 inch). I suspect the former value is more likely than the latter. Females are also reputed to have 32 ovaries.

Every living thing on the planet, with the possible exception of bacteria, other nematodes and some protists such as amoebas, is parasitized by nematodes. For example, some nematodes are:

  • totally free-living,

  • free-living as juveniles and parasitic as adults,

  • parasitic as juveniles and free-living as adults,

  • totally parasitic, but with each life stage in a different host or,

  • totally parasitic with each life stage in the same host. Then, add to this the fact that some are parasitic in plants at one stage, and in animals during another.

Thus, if an estimated 10,000,000 animal species are on the planet (exclusive of nematodes) and if each has at least one unique nematode parasite (a conservative number, humans have many...), then there are at LEAST as many nematode species as all other animal species combined. And this doesn't include those that are parasitic in more than one host, or in plants, or the free-living forms.

I should point out as well that you, as you read this, have nematodes living in your tissues and probably in your gut. Such knowledge gives a whole new meaning to the quote from Queen Victoria, who upon hearing of some foible of a cabinet minister, replied, "We are not amused." While I presume she could speak for herself, I rather think her worms could form their own opinions.

Nematodes On and In Reefs

Free-living nematodes occupy just about every ecological niche. Some are predators, some are herbivores, while others scavenge detritus for nutrients. All three types are likely to be found commonly in our aquaria. Judging from the gut contents I have seen in the animals in my systems, the scavengers predominate, but more observations need to be made before anything definitive should be concluded. Predatory nematodes are common in some marine systems where they eat foraminiferans, clams or other small shelled prey. Herbivorous nematodes may be found in algal films or around clumps of filamentous algae. These worms often suck the cellular contents out of algal cells. In such environments the nematodes themselves are likely prey for larger organisms such as some crustaceans. Scavengers are common in live sand beds where they scrape bacteria off mineral grains or eat pieces of detritus.

In our systems, as in nature, nematodes perform several important roles, acting as predators, herbivores and scavengers in a size range that is normally underrepresented in our systems. They are really larger than most protozoa, but smaller than most annelid worms. Consequently, they are more important to our systems than their small size would imply, as they facilitate the transfer and utilization of nutrients through a potential bottleneck in the system.

Figure 6. An unidentified, about 1 cm (0.4 in) long, nematode collected from one of my aquaria. The sensory bristles around its mouth are indicated. Nematodes that are long relative to their girth, such as this one, tend to coil when removed from their normal habitat and placed into water. This animal appeared to be eating microalgae found in my system's deep sand bed.

Aquarists can't do much to encourage the growth and well being of their systems' nematode fauna. Provided with a fine-grained sand bed, however, the nematodes will normally thrive. Treatment of the system with some medications, specifically those for roundworm parasites, should be avoided, as they will seriously impact the beneficial worms. If you are concerned about a fish having parasitic nematodes, treat the fish in a quarantine or hospital tank, not your reef system.

If you have any questions about this article, please visit my author forum on Reef Central.

Some References:

Aleshin, V. V., O. S. Kedrova, I. A. Milyutina, N. S. Vladychenskaya and N. B. Petrov. 1998. Relationships among nematodes based on the analysis of 18S rRNA gene sequences: molecular evidence for monophyly of chromadorian and secernentian nematodes. Russian Journal of Nematology 6:175-184.

Blaxter, M. L., P. De Ley, J. R. Garey, L. X. Liu, P. Scheldeman, A. Vierstraete, J. R. Vanfleteren, L. Y. Mackey, M. Dorris, L. M. Frisse, J. T. Vida and W. K. Thomas. 1998. A molecular evolutionary framework for the phylum Nematoda. Nature 392:71-75.

Blaxter, M. L., M. Dorris and P. De Ley. 2000. Patterns and processes in the evolution of animal parasitic nematodes. Nematology 2:43-55.

de Ley, P. 2000. Lost in worm space: phylogeny and morphology as road maps to nematode diversity. Nematology 2:9-16.

Dorris, M., P. De Ley and M. L. Blaxter. 1999. Molecular analysis of nematode diversity and the evolution of parasitism. Parasitology Today 15:188-193.

Gubanov, N. M. 1951. Giant nematoda from the placenta of Cetacea; Placentonema gigantissima nov. gen., nov. sp. Doklady Akademia Nauk SSSR.;77(6):1123-5.

Kampfer, S., C. Sturmbauer and J. Ott. 1998. Phylogenetic analysis of rDNA sequences from adenophorean nematodes and implications for the Adenophorea-Secernentea controversy. Invertebrate Biology 117:29-36.

Kozloff, E. N. 1990. Invertebrates. Saunders College Publishing. Philadelphia. 866 pp.

Litvaitis, M. K., J. W. Bates, W. D. Hope and T. Moens. 2000. Inferring a classification of the Adenophorea (Nematoda) from nucleotide sequences of the D3 expansion segment (26/28S rDNA). Canadian Journal of Zoology 78:911-922.

Malakhov V. V. Nematodes. Structure, development, classification, phylogeny (ed. W. Duane Hope). Smithsonian Institution Press. Washington and London. 1994. 286 pp.

Nielsen, C. 1998. Systematics - Sequences lead to tree of worms. Nature 392:25-26.

Poinar, G. O. 1983. The Natural History of Nematodes. Prentice Hall, Englewood Cliffs, NJ. 323 pp.

Ruppert, E. E, R. S. Fox and R. D. Barnes. 2003. Invertebrate Zoology, A Functional Evolutionary Approach. 7th Ed. Brooks/Cole-Thomson Learning. Belmont, CA. xvii +963 pp.+ I1-I26pp.

Voronov, D. A., Y. V. Panchin and S. E. Spiridonov. 1998. Nematode phylogeny and embryology. Nature 395:28-28.

Wright, K. A. 1991. Nematoda. Pages 111-195 In: Microscopic Anatomy of Invertebrates, Vol. 4. F. W. Harrison and E. E. Ruppert, eds. Wiley-Liss, New York.

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Nematodes by Ronald L. Shimek, Ph.D. - Reefkeeping.com