Nematodes

We will follow the classification scheme utilized by Roberts and Janovy. The classification of nematodes is another subject which has changed rapidly in the last decade. When you encounter terms which are unfamiliar refer to your glossary for the definitions.

General Features

As is clear from the statement by N.A Cobb at the start of this chapter, nematodes are the most abundant organisms on earth.  They have been known for as long as humans have recorded their history.  They are found as parasites of plants and animals and many species are free-living at some point or throughout their life cycle.

The typical shape of a nematode is an elongated worm with tapered ends.  They have bilateral symmetry and the body cavity is a pseudocoelom that arises from the blastocoel.  Figure 22.1 illustrates some of the variations on this plan.  Unlike the flatworms, the nematodes have a complete digestive system with an anterior mouth and a posterior anus (Figure 22.2).  A triradiate pharynx is characteristic of the nematodes (Figure 22.19).  A noncellular cuticle (Figure 22.3) covers the body and is produced by an underlying hypodermis.  The cuticle is molted four times during development.  All body wall muscles are longitudinal in arrangement and their contraction gives nematodes the typical whiplike movement that characterizes their locomotion.  The excretory system is made up of later and or/ventral glands that open at an anteroventral excretory pore.  Species with separate sexes are much more common than hermaphroditic species and sexual dimorphism is frequently seen with females the large sex.  Males often differ from females in having a curved tail end.  Some hermaphroditic and parthenogenetic species do exist.  Oviparity is more common than ovoviviparity.  The male system opens in a cloaca along with the digestive tract while the female system has a separate ventral genital pore.  Length varies considerably; Dracunculus may be over one meter long while Caenorhabditis is less than 1 mm.

Body Wall

The cuticle is complex and very important to the proper functioning of the animal.  It is the outermost covering and also lines several openings such as the proctodeum, stomodeum, excretory pore, and vagina.  The cuticle is covered by a carbohydrate-rich surface coat that seems to be important in resisting the host's immune response in parasites of animals.

The cuticle has three layers; the cortical layer, the middle or homogeneous layer, and the inner fibrous layer.  Each of these layers can be further subdivided (Figure 22.3).  The cortical layer has a thin epicuticle, an external cortical layer, an internal cortical layer, and a fibrillar layer. Cuticlin, a highly resistant protein, is found in the external layer.  Collagen is abundant in the internal layer and in the other layers of the cuticle.  The homogeneous (middle) layer has no apparent structure even under the electron microscope.

The fibrous layers are two or three collagenous layers.  The first and third layer run at about 75 degree to the longitudinal axis, while the middle layer runs at about 135 degrees to the other two layers.  These layers are important parts of the hydrostatic skeleton of larger nematodes.  The basal lamella is the innermost layer of the cuticle and consists of fine fibrils that merge with the hypodermis.  Ringlike depressions in the cuticle enhance its flexibility.

Numerous cuticular markings and structures exist.  Punctuations are shallow depressions, pores are deeper depressions, and spines are self-explanatory.  Alae are lateral or sublateral cuticular thickenings found in many species.  Some species have lateral alae that extend the length of both sexes; caudal alae are found at the tail end of some males; and cervical alae (Figure 22.4) are found in the anterior region.  Lateral alae may assist in locomotion.  Many species have longitudinal ridges (Figure 22.6) that aid locomotion (Figure 22.7).  The cuticular ridges are supported by skeletal rods termed the synlophe.

The hypodermis lies just below the basal lamella of the cuticle.  In adults, it is usually syncytial with the nuclei in four thickened portions (six to eight in some species) called the hypodermal cords.  These cords run longitudinally and divide the musculature into four quadrants.  On some large nematodes these cords are visible on the outside as pale lines.  The lateral cords contain canals of the excretory system, in many species, and the dorsal an ventral cords contain nerve trunks.  The hypodermis secretes the cuticle and contains mitochondria and endoplasmic reticulum (ER) abundantly in the areas of the cords.

Musculature

The somatic muscles, pseudocoel, and cuticle function together as a hydrostatic skeleton in nematodes.  The platymyarian type of muscle cell has contractile fibers on one side, next to the hypodermis, and a noncontractile myocyton bulging into the pseudocoel (Figure 22.8).  The myocyton contains the nuclei and a variety of organelles such as mitochondria, ribosomes, and ER and glycogen and lipid molecules.  A second type of muscle cell arrangement is the coelomyarian cell which is more spindle shaped.  The contractile portion of the coelomyarian cell is U-shaped and located near the distal end (Figure 22.9).  Mitochondria are packed in the distal portion of the cell near the contractile fibrils.  The myocyton bulges located medially into the pseudocoel and contains mitochondria, ER, a Golgi apparatus, and abundant glycogen.  The circomyarian cell type has its contractile fibrils at the periphery encircling the myocyton.

Muscle contraction seems to be similar to the sliding filament model seen in vertebrate muscles.  As in the vertebrates, thick myosin and thin actin fibers are involved and striations are visible, although the myofilaments are slightly offset and appear obliquely striated (Figure 22.9).

Nerve processes run from the myocytons to the nerves (Figure 22.10) as opposed to the opposite direction of processes seen in vertebrates.  In the coelomyarian muscles type, there are many muscle-muscle interconnections that appear to aid in coordinated contraction by ensuring nerve impulse transmission.

Pseudocoel and Hydrostatic Skeleton

The fluid-filled pseudocoel (pseudocoelom) (Figure 22.11) arises from the embryonic blastocoel rather than as a cavity in embryonic mesoderm as is seen in coelomic animals.  Hence, a pseudocoel lacks the mesodermal lining seen in animals with a true coelom.  The hydrostatic skeleton works when the muscles contract and apply pressure to the incompressible fluid in the pseudocoel and this pressure is transmitted in all directions in the fluid.  The longitudinal muscles act against the cuticle and the combination of muscle contraction, pressure on the fluid and stretching and compressing of the cuticle give nematodes their characteristic whip-like locomotion.

Locomotion is achieved through the following activities: the ventral or dorsal muscles contract and compress the cuticle on that side.  The contractile force is transmitted to the opposite side (either dorsal or ventral) to cause the cuticle on that side to stretch.  Compression and stretching of the cuticle antagonize the muscle and force the body to return to the resting position when the muscles relax.  Alternate contraction and relaxation of the ventral and dorsal muscles move the nematodes.  The hydrostatic pressure in Ascaris pseudocoels ranges from 70 to 120 mm Hg and can go as high as 210 mm Hg.  This increase in hydrostatic pressure increases the efficiency of locomotion.  These hydrostatic pressures are much higher than those seen in other invertebrates that have hydrostatic skeletons.  The high hydrostatic pressure also influences the morphology and physiology of nematodes.

The pseudocoelomic fluid is called hemolymph and in many nematodes it is a clear, pink, complex solution that is almost cell-free.  It probably serves to transport solutes from tissue to tissue.  One type of cell that is found in the pseudocoel is the coelomocyte.  Two, four, or six of these cells are found and they have an ovoid shape with many branches.  They attach to surrounding tissues.  They are usually small, but in Ascaris they can be 5 mm by 3 mm and 1 mm thick.  Their functions are unclear.

Nervous System

Morphology

Longitudinal nerve trunks connect the nerve concentration in the esophageal region to the nerve concentration in the anal region.  The anterior nerve concentration is the nerve ring or circumesophageal commissure.  The ring is a commissure for the dorsal, ventral, and lateral ganglia which are usually in pairs.  The longitudinal nerve trunks arise from these ganglia (Figure 22.12).  The ventral ganglia and the ventral nerve trunk are the largest examples of each type of structure. Amphidial nerves proceed anteriorly from the lateral ganglia. Papillary nerves arise directly from the nerve ring and innervate cephalic sensory papillae around the mouth.

The ventral nerve cord is a chain of ganglia running posteriorly.  The last ganglion is the preanal ganglion that give rise to two branches that pass dorsally into the pseudocoel to encircle the rectum forming the rectal commissure or posterior nerve ring (Figure 22.12).  Peripheral nerves form a latticework of fibers that join with small commissures to supply nerves endings to sensory structures in the cuticle.

In parasitic species, the main sense organs are amphids, phasmids, cephalic papillae, and  caudal papillae.  Arrangement of cephalic nerves suggests that sixteen papillae existed in primitive nematodes and sixteen nerve endings can be found in modern nematodes even when far fewer papillae exist.  The arrangement of papillae in nematodes is a strong taxonomic character.  Sensory endings of papillae appear to be modified cilia and the papillae are probably sensory receptors.

Amphids are a pair of more complex sensory organs that open laterally at about the same level as the papillae.  Amphids are reduced in parasitic nematodes.  A deep cuticular pit leads from the amphid opening to nerve processes in a nerve bulb (Figure 22.14).  Modified cilia serve as the sensory endings.  Amphids are thought to be chemoreceptors. Hookworm amphids secrete a substance that prevents clotting of vertebrate blood to aid in their feeding.

A pair of cervical papillae or deirids are found in many parasitic nematodes at the level of the nerve ring.  These and other sensory papillae are found along the body length of many nematodes. Caudal papillae (Figure 22.15) are very elaborate in males and help in copulation.  Distribution of caudal papillae is an important taxonomic feature.

Phasmids (Figures 22.16 & 22.17) are found in some species.  These are similar to amphids, but have fewer nerve endings.  If a gland is present, it is smaller than those found in amphids.

Neurotransmission

Acetylcholine (ACh) is the primary excitatory neurotransmitter in nematodes.  Release of ACh at the neuromuscular junction will increase the rate of spikes (action potentials, Figure 22.18).  Inhibitory fibers release gamma-aminobutyric acid (GABA), that hyperpolarizes the muscle to decrease the action potentials.

Other neuropeptides have been discovered, but their functions remain unknown at present.  Serotonin has been found and may work in conjunction with ACh and neuropeptides to control and modulate feeding activities.

Effects of Drugs

Piperazine hyperpolarized the muscle membrane to effectively paralyze nematodes so they pass out of the host.  Levamisole and pyrantel mimic the effects of ACh to depolarize the membrane which also paralyzes the worm.  Ivermectin appears to inhibit esophageal pumping so that the nematodes cannot take in adequate nutrients.  Mebendazole, parbendazole and fenbendazole appear to inhibit mitochondrial electron transport to inhibit energy production.  They also bind to tubulin so that microtubule-dependent processes are paralyzed.

Digestive System

Most nematodes have a complete digestive system.  The stomodeum consists of the buccal cavity and esophagus and it is lined with cuticle.  The proctodeum (rectum) is also lined with cuticle.

Mouth and Esophagus

A maximum of six lips surround the round mouth opening.  Few parasitic species have six lips and often they are fused into three lips.  Other species lack lips entirely and other modifications exist.

The buccal cavity leads from the mouth to the esophagus.  It varies with species and is an important taxonomic characteristic.  The cuticular lining of the buccal cavity may be thin or thick (buccal capsule).  The buccal cavity may be elongated, reduced, or missing.  Buccal armament such as teeth may form from the cavity wall or anterior part of the esophagus.  Hookworms have such teeth.

Food is sucked from the esophagus or pharynx into and through the intestine. This sucking is necessary to force food through the intestine against the high pressure found in the pseudocoel.  Esophageal structure varies with species and is another important taxonomic character.  The esophagus is lined with cuticle and triradiate in shape (Figure 22.19).  Muscles insert on the cuticle of the radii.  Glands are found among the muscles and they open into the lumen of the esophagus.  The dorsal gland is usually the most extensive (Figure 22.19).  The esophageal glands secrete digestive substances such as the enzymes amylase, proteases, cellulases, chitinases, and pectinases.  Hookworms secrete anticoagulants with these glands.

Figure 22.21 illustrates the function of the esophagus during feeding.  The sucking pressure created is important in food ingestion.  The posterior bulb serves as a nonregurgitation valve.  Figure 22.22 illustrates some variations on the theme of esophageal structure in the ascaroids.

Intestine

The intestine stretches from the esophagus to the proctodeum and is a simple, tubelike structure.  It is made of a single layer of cells.  Female nematodes have a short, cuticle-lined rectum between the anus and the intestine.  The male rectum receives reproductive tract products and is thus considered a cloaca.  The dorsal wall of the cloaca is invaginated into two pouches called the spicule sheaths.  Copulatory spicules are in these sheaths.  The vas deferens opens into the ventral wall of the cloaca.  There are no muscle in the intestine to move food along.  As more food enters its anterior end, the food already present will pushed posteriorly.  The depressor ani muscle dilates the anus and this muscle would be better named the dilator ani.  When the anus opens, hydrostatic pressure forces the waste material out (defecation).

The mucosa of the intestine is a typical columnar epithelium (Figure 22.23) lined with a brush border of microvilli at the luminal surface.  These microvilli increase the absorptive surface area of the epithelial cells.  In addition to absorbing nutrients, the epithelial cells probably excrete nitrogenous wastes.

Food

Blood, tissue and cells, intestinal contents or combinations of these serve as the food of parasitic nematodes.  Nematodes take in a great deal of food, much of which is never utilized.  Hence, a severe hookworm infection can lead to anemia of the host.

Secretory-Excretory (SE) System

It is thought that most actual excretion occurs through the intestine and that the so-called excretory systems were named as such based merely on their morphology and before definitive functions were identified.  Most excretory systems appear to have secretory and osmoregulatory functions and can be termed S-E systems.

Two basic types of S-E systems exist: glandular and tubular.  Most free-living nematodes have the glandular type while the tubular type is commonly found in the parasitic nematodes.

Figure 22.24 illustrates the types of tubular systems that exist.  The most common arrangement is two long canals in the hypodermis connecting to each other by a transverse canal near the anterior end.  A median, ventral duct or pore opens from the transverse canal to the outside.  This is the excretory pore.  Its location is species specific and serves as a taxonomic character.

Osmoregulation

The particular habitat of a species will determine its osmoregulatory abilities.  Ascaris suum lives in the gut of pigs and maintains a hemolymph osmotic pressure about 80% to 90% that of the pig's gut contents.  The pig's intestinal chloride concentration varies between 34 and 102 mM, but Ascaris maintains its hemolymph at 52 mM chloride.

Water and ion excretion are poorly understood in nematodes.  The S-E systems seem to be involved in water excretion, but the details remain mysterious.

Secretion

The gland cells associated with the S-E system has an ultrastructure that suggests secretory function.  Some products secreted by these cells in various species include exsheathment enzymes, substances antigenic to the host's immune system, and digestive enzymes.

Excretion

Ammonia is the major nitrogenous waste product of nematodes.  Urea is a lesser component of the nitrogenous wastes.  Amino acids, peptides, and amines may also be excreted by nematodes.  Other types of excretory products include carbon dioxide and several fatty acids.  The cuticle is also thought to play a role in ammonia secretion in most nematodes.

Reproduction

Most nematodes are dioecious, but some are monoecious and a few parthenogenetic forms are known.  Dioecious species exhibit sexual dimorphism with females being larger than males.  Males have a curled tail and more external ornamentation such as bursae, alae, and papillae (the males have more "jewelry").

Nematode gonads are solid cords of cells continuous with the oviducts and uteri leading to the external environment.  If the ducts were not continuous, the hydrostatic pressure would squeeze them shut and the ova could not pass from the ovary to the oviduct.  Germ cell proliferation at only one end of a gonad is termed telogonic; germ cell proliferation throughout the gonad is termed hologonic.

Female System

Two ovaries are common in nematodes, but the range is from one to six.  The gonopore is independent of the digestive tract (unlike the situation in males).  The structures arranged in a linear fashion from the proximal end to the ejective end which is distal.  Most female gonads are telogonic.

Ovaries and Oviducts

Ovaries are solid cords of cells that produce oocytes and move them along towards the oviducts.  The proximal end of the telogonic ovary is the germinal zone that produces oogonia.  These become oocytes that move along to the distal growth zone (Figure 22.30) of the ovary.  Large ascarids have a central supporting structure called the rachis to which the oocytes attach.  They increase in size as they move along the rachis toward the oviduct and detach just before the oviduct.

The proximal end of the oviduct in most nematodes is a sperm storage area called the spermatheca.  Sperm will penetrate the oocytes as they enter the spermatheca.  Meiosis will go to completion after sperm entry and two polar bodies will be ejected.  Egg shell formation also occurs after fertilization.

Uterus and Vulva

The wall of the uterus has both circular and longitudinal muscles that create peristaltic waves to move the embryos along.  The uterus may mold the shape of the eggs containing the embryos and uterine secretions may add to the eggshells.  The distal end of the uterus is quite muscular and forms an ovijector.  The ovijectors fuse to form a short vagina that opens through a ventral, transverse slit in the body wall called the vulva.  Location of the vulva varies by species from near the mouth to near the anus.

Male System

Testes and Ducts

Some groups have paired testes while others have a single testis.  In larger nematodes the testes are long and threadlike and coil around the intestine and themselves, while smaller nematodes have short, uncoiled testes.  Telogonic testes (Figure 22.26) have a germinal zone in which spermatogonial division occurs and a growth zone for maturation.  The distal end of the growth zone joins with the seminal vesicle as a sperm storage area.  Following the seminal vesicle is the vas deferens which has the following components: anterior, glandular region and a posterior, muscular ejaculatory duct.  This duct empties into the cloaca.  Some species have cement glands that produce a plug to block the vulva after copulation.

Accessory Organs

Copulatory spicules (Figure 22.27) are common accessory organs in most species of nematodes.  The spicules are each surrounded by a spicule sheath and spicule size is species specific making them good taxonomic characters.  In some species, a dorsal, sclerotized structure called the gubernaculum exists to help guide the spicules out the cloaca opening during copulation.  In some species a ventral telamon serves the same purpose.  Spicules are inserted into the vulva at copulation to hold it open so that pressure developed by the ejaculatory muscles can overcome the high hydrostatic pressure and the sperm can be rapidly injected.

Spermatozoa

Nematode spermatozoa (Figures 22.26, 22.28 & 22.29)  lack a flagellum which makes them unusual in the animal kingdom.  Among the various species, the sperm morphology varies and at least four types have been described.  The internal organization is also unusual in that there is no true nuclear membrane.  Lack of a flagellum would seem to limit locomotion, but some sperm can become ameboid to move up the female tract.
 

Mating Behavior

Chemotaxis and thigmotaxis are utilized to allow worms of the opposite sex to find each other in their habitats.  Many species are known to produce pheromones, at least in the laboratory.  The exact identity of these molecules remains unclear in many cases.

Once the female and male have found each other thigmotactic responses mediated by sensory papillae aid in copulation.  The female in some species seeks the coiled end of the male which she enters.  The caudal papillae of the male detect the vulva and a probing response allows spicular insertion followed by sperm transfer.  Females may wander in search of any constriction when males are lacking leading to some curious results (see Figure 22.31).
 

Development

Early studies on nematode development led to discoveries that were also applicable to other organisms.  For example, meiosis was first worked out by studies done on nematodes.  The genetic continuity of chromosomes and determinate cleavage were also first discovered in nematodes.

In spite of the great variability in life cycles among the nematodes, they all exhibit the same general pattern.  There are four juvenile stages and an adult stage with four molts or ecdyses during development from the first-stage juvenile to the adult.  Each juvenile stage is similar to the adult and usually possesses the same somatic cells as the adult.  They seem to change in size with each stage and the germ cells will develop.

Eggshell Formation

The eggshell consists of three layer in most nematodes: (1) and outer vitelline layer, may not be detectable with a light microscope; (2) a chitinous layer; and (3) a lipid layer innermost.  Some nematodes have a fourth layer, the proteinaceous layer, which consists of mucopolysaccharide and tanned protein in a complex.

After sperm penetration, a new plasma membrane forms beneath the original membrane and the old membrane become the vitelline layer and separates from the peripheral cytoplasm leaving a clear space where the chitinous layer can form (Figure 22.32).  Refringent granules or bodies move to the periphery of the cytoplasm to secrete their contents that form the lipid layer.

The chitinous layer probably provides strength, support,  and protection and also contains protein.  The lipid layer resists desiccation and penetration by polar molecules.  The lipid layer of ascarids have interesting molecules called ascarosides that resist entry by all molecules except lipids and gases.

Embryogenesis

Nematodes exhibit determinate cleavage and nematode development has been well studied.  The fate of each blastomere is determined early during cleavage and this has allowed researchers to mark and map the fate of each cell to learn which tissues will develop from a given cell.  This is illustrated in Figure 22.33.  During early cleavages chromatin diminution occurs in which the ends of the chromosomes break off and degenerate.  It is not known why this occurs or what functions may be served.  This phenomenon occurs only in the somatic cells, so that the germ line cells can be easily distinguished by the presence of intact chromosomes.

The common developmental stages of morula, blastula, and gastrula form.  Gastrulation occurs by invagination and epiboly (the micromeres move down and over the macromeres).

Once the embryo is fully formed the nuclei in the somatic cells cease to divide and all of the cells that will be present in the adult are now present.  This is termed cell constancy, nuclear constancy, or eutely.  Other pseudocoelomate phyla also exhibit this phenomenon.

Embryogenesis occurs at different times in different places among nematode species.  Some complete embryogenesis before the egg passes from the host; others have development of the juvenile and hatching within the female (ovoviviparity); while still others will not undergo embryogenesis until the zygote reaches the external environment.

Embryonic Metabolism

Embryonic metabolism is probably best known from Ascaris and the following will be based on this knowledge.  Metabolic pathways are turned on and off as needed through development as genes are repressed and derepressed.

Whereas adult Ascaris utilize anaerobic metabolism, juveniles in the embryonating egg are obligatory aerobes.  If they utilized glycolysis, they would build up toxic acidic end products  within the eggshell.  Embryogenesis is complete to the second-stage juvenile within 20 days at 30 C.  They are infective at that stage.  During development to the infective stage, Krebs cycle and an electron transport system are used for energy production.

After entering the gut of the host, the juvenile hatches and goes through a tissue migration.  It enters the lung alveoli, travels up the trachea, is coughed into the pharynx, is swallowed, passes through the stomach, and enters the intestine.  In the intestine molting through any additional juvenile stages to the adult will occur.  Juveniles recovered from the lungs are still aerobic, but fourth-stage juveniles in the intestine lose the ability to metabolize aerobically.

Hatching

Some nematodes have free-living juveniles that hatch spontaneously before the parasitic stage of the life cycle.  Many parasitic nematodes must be swallowed to hatch in an animal host.  In these latter species, the juvenile will remain dormant until appropriate stimuli are encountered, which prevents premature hatching.  Ascarid eggs require the following stimuli to hatch:

Not surprisingly, these are the conditions found in the mammalian intestine and even some birds where Ascaris will hatch.  The egg will take in water and the increased concentration of water will activate the juvenile which will secrete a variety of enzymes to help break down the eggshell.  Once the eggshell is sufficiently weakened, the juvenile will escape.

Growth & Ecdysis

Molting

Unlike arthropods, nematodes will grow between molts and the cuticle will grow to accommodate the increase in size (Figure 22.34).  The first step in molting is separation of the hypodermis from the basal lamella of the old cuticle and beginning of secretion of a new cuticle.  Significant new cuticular growth may occur prior to ecdysis.  A collagenaselike enzyme appears to aid destruction of the old cuticle and escape.

Developmental Arrest

Developmental arrest or hypobiosis serves as a resting stage that allows many species of nematode to withstand unfavorable conditions until a host can be found.  Many parasitic nematodes produce an infective third-stage juvenile that is comparable to the resting-stage dauer larva found in free-living nematodes.  These third-stage juveniles live within the old second-stage cuticle and survive on stored food reserves.  Some third-stage juveniles will exhibit behavior that enhances the odds of their encountering a host (see Figure 22.35).

Specific stimuli are required to allow these third-stage juveniles to develop further and escape the second-stage cuticle.  Those that penetrate skin (hookworms) will exsheath as they pass through the skin.  Third-stage juveniles that are swallowed will exsheath in response to stimuli that are similar to those involved in hatching of Ascaris eggs.  Carbon dioxide seems to be one of the most important stimuli for exsheathment in those nematodes that have been studied.

Nematodes that utilize intermediate hosts will undergo hypobiosis until they encounter stimuli that indicate they are in the proper definitive host.  Some will become dormant in older female animals until hormones from the host indicate pregnancy and they will then migrate to the uterus or mammary glands to infect the young either in utero or when feeding on milk after birth.  Many other examples of hypobiosis can be found in the chapters on specific life cycles.

Metabolism

Energy Metabolism

Adults

Some of the original work on metabolic processes was done on nematodes.  The cytochromes that are part of electron transport systems were first discovered in nematodes.  Most of this work and the work that followed was done on Ascaris, but it is assumed that similar processes are used by other adult nematodes.

The metabolic processes of adult nematodes are similar to those seen for trematodes and cestodes.  They do not completely oxidize their nutrient molecules, but some components of the Krebs cycle and electron transport system seem to be used.  For example, glucose is degraded to phosphoenolpyruvate and fix carbon dioxide to form oxaloacetate (Krebs intermediate) which is reduced to malate (Krebs intermediate).  The malate enters the mitochondrion where additional ATP is produced, but products such as lactate, acetate, and succinate are produced which does not normally happen with complete oxidation of nutrients.

There is no evidence of a complete electron transport system and oxygen in moderate amounts is actually toxic to Ascaris.  Many of the aspects of metabolism remain unknown.

Other nematodes seem to have an alternate system that can be used in the presence of oxygen.  These organisms have an Ascaris type of system for anaerobic conditions and utilize a Krebs cycle system when oxygen is present.  This helps resolve the problem reoxiding NADH in the absence of oxygen as a terminal electron acceptor.

At the other extreme, some nematodes are obligate aerobes and require at least some oxygen to survive.  These organisms have a classical cytochrome system (electron transport system), but still do not completely oxidize their nutrients and a variety of end products are produced.

Some authorities say that size correlates with type of metabolism.  Larger nematodes tend to be anaerobic, while smaller nematodes can get some oxygen near the mucosa and are aerobic.

Juveniles

Juveniles nematodes have a variety of metabolic adaptations from aerobic to anaerobic.  The adaptations are associated with the life cycles exhibited by the various species.  Some species have juveniles that are anaerobic for two stages and then the third-stage juvenile is aerobic. Ascaris juveniles are aerobic until the third-stage when they switch to anaerobic.  Other species seem to combine aerobic and anaerobic metabolic capabilities.  No generalizations can be made about juvenile nematode metabolism.

Synthetic Metabolism

Proteins & Nucleic Acids

Ascaris has been studied more than most nematodes with respect to protein and nucleic acid synthesis.  These processes must work well because Ascaris can produce an enormous number of offspring requiring concomitant protein and nucleic acid synthesis.  In oocytes, cytoplasmic RNA (rRNA) is present in large quantities during synthesis of yolk proteins, but disappears after the synthesis stops.  The sperm contains little or no RNA, but immediately after fertilization, massive amounts of rRNA are produced in the male pronucleus.  This occurs as the female pronucleus is finishing its meiotic divisions.  It has been suggested that the ribosomes coded for by the male genome are responsible for shell formation and cleavage.  Amino acids utilized in protein production are probably obtained by intestinal absorption, but some species appear to be able to synthesize a variety of amino acids from such substrates as acetate.

Lipids

Little is known of lipid synthesis.  Some nematodes can synthesize polyunsaturated fatty acids but are unable to synthesize sterols. Ascaris can synthesize some lipids and Dirofilaria immitis can synthezise all types of complex lipids, including cholesterol.
 
 

Classification of Nematodes (see pp. 379-381 and specific chapters in Roberts & Janovy)

Keep in mind that these taxa are subject to change and reflect the views of some, but not all, parasitologists. We will focus on the parasitic groups.

Phylum Nematoda

Life Cycles

You should be familiar with the life cycles studied in lab and any others that hold particular interest to you.