Nematode section labeled
Nematode section
Above a 3D computer model of a nematode (round worm). This model has been based on the fresh-water
nematode
Ethmolaimus. Nematodes are generally colourless/white and translucent, but here the internal organs
have been coloured for clarity. Many nematodes of parasites with species parasitising many different organisms,
plants and animals, including humans. Free-living forms are mostly tiny, often microscopic, and may be found in
water or damp soil. In water nematodes swim by a graceful eel-like motion as they throw their stiff but elastic bodies
into sinusoidal curves by contracting longitudinal muscles (the elasticity of the body returns it to its original shape).
A computer model illustrating a transverse section
through a nematode. C, cuticle; DN, dorsal nerve;
EC, excretory canal enclosed by the lateral nerve
cord; H, hypodermis; LM, longitudinal muscles; N,
nerve extending from muscle cell to main nerve; P,
pharynx; VN, ventral nerve. Note the triradial
arrangement of the pharynx, a feature which gives
this organ greater mechanical efficiency.
Nematoda (Nemata, Round Worms)

External Features

Most nematodes are < 2.5 mm long, and are often microscopic, however, some marine species reach 5 cm.
Dioctophyme renale, the kidney worm, and Dracunculus medinensis, the guinea worm, may reach 1 m in length.
Nematodes are of circular cross-section and fusiform or filiform worms. The posterior end is usually curved in the
male in which it may bear papillae and wing-like
alae. Nematodes are transparent, whitish or yellowish (due to
cuticle). There are no definite regions and no distinct head. In the family Draconematidae, a constriction
demarcates the anterior end, including the entire pharyngeal region.

There is a midventral anterior excretory pore, a midventral and posterior female gonopore (or vulva) and a
posterior midventral anus. The postanal region forms the tail.

The mouth is in the centre of the anterior tip and may be surrounded by 6 liplike lobes in primitive marine forms,
three on each side, but there are often only three lips in terrestrial and parasitic species, as a result of fusion.
Primitively the lips bear 16 sensory papillae or setae. Small interlabial lobes may also be present. A circular groove
separates the labial region from the anterior tip. The anterior end has a hexaradial-biradial structure.

Probolae, or rigid lip protuberances, occur in certain terrestrial nematodes, e.g. Acrobeles and Wilsonema. There
are 3 or 6 (2 circles of 3) of these probolae. These probolae are forward projecting, rounded, conical, forked or
antler-like projections, movable only by lip motions, and of unknown function.

In many Strongylidae the lips form an upstanding collar, with up to 40 or more lobes or teeth on the inner surface,
forming the leaf crown (corona radiata) and there may be a similar additional inner leaf crown.

In the order Spiruroidea (the spirurine nematodes) the lips are encircles by a cuticular collar, which may be
extended dorsally and ventrally, to form head shields. Some members of the spirurine family Physalopteridae have
an upstanding collar or collarette into which the anterior end can withdraw. In the spirurine family Acuariidae have 2,
4 or 6 anterior longitudinal cuticular chord-like thickenings or grooves, called cordons or epaulets, which may be
straight, recurved or looped, or horseshoe-shaped. In Seuratia and some oxyuroids the free posterior edge of the
cordons forms spines and there are 4 cordon scallops. Heterocheilus has a cuticular thickening behind the lips.
Typhlophorus has a similar thickening with longitudinal ribs.

In several spiruroid genera, appendages project from the head. There are 4 feathery projections in
Ancyracanthus;
4 pointed wings in
Schistorophus; 8 simple lobes in Ancyracanthopsis; 2 split lobes, giving rise to secondary lobes
in
Histiocephalus and a circle of variously shaped processes in Serticeps.

In the spiruroid family Gnathostomidae, 4 cuticular inflations or ballonets form a swollen band or head bulb behind
the lips. This may be armed with circlets of spines or transverse striations.

Spines, warts and longitudinal ridges may be present. These ridges may form 1,2 or 4 wings or alae. These alae
may be cervical or caudal (latter involved in copulation). Some nematodes have bristles, for example the stilt bristles
or ambulatory setae, used in locomotion in Epsilonematidae and Draconematidae. The body sense organs are
concentrated in 4 or 8 longitudinal rows.

In many free-living nematodes adhesive caudal glands open via a pore at the posterior tip. This pore may be
mounted on an adhesive tube (similar to that structure seen in
rotifers and gastrotrichs).
Body Wall

The outer cortical layer is bounded externally by a thin epicuticle, which may be quinone-tanned and is typically
ringed / annulated. The median layer consists of struts, skeletal rods, fibrils, canals or is uniform and granular. The
basal layer is striated, laminated, or may contain spiral fibres.

The outermost layer is the
epicuticle and is 6-45 nm thick. This consists of a 3-layered osmiophilic membrane,
which may have an outer glycocalyx. Inside this membrane there may be an inner osmiophilic layer. There is a more
or less clear layer between these two osmiophilic membranes. Annulations are transverse grooves in the epi- and
exocuticles and in
Enoplus they reach the mesocuticle cavity which may open to the outside at places. Pores or
striations may also cross the epicuticle. Scales or longitudinal ribs may be present.

Beneath the epicuticle is the
exocuticle, which varies in thickness from 0.2 m in small, free-living forms to ~ 10
m in large, parasitic forms. The exocuticle is, however, absent in some small Rhabditia,
Monoposthia, Ascaris and
Oxyuris. The exocuticle consists of striations perpendicular to the surface, composed of crystalline protein. The
periodicity of these striations is 10-17 nm in longitudinal section and 20-27 nm in transverse section. The exocuticle
either closely adjoins the epicuticle or is separated from it by a thin layer of homogeneous material or, in Rhabditia,
by a thick homogeneous layer or longitudinal fibrous material.

Beneath the exocuticle is the
mesocuticle. This is usually the thickest layer of the cuticle, and is the most varied
component, but is sometimes reduced. The mesocuticle may contain an intracellular cavity, which is fluid-filled or
filled with loose or porous material and is intersected by columns of dense osmiophilic material supporting the epi-
and exocuticles. In
Paracanthonchus the cavity accounts for 75-80% of the cuticle thickness. In Sphaerolaimus the
cavity is filled with a coarsely alveolar osmiophilic mesh. The mesocuticle usually also contains 2-3 layers of oblique
protein fibres at 60-75o to the longitudinal axis. These form spirals in opposite intertwining directions around the
worm. These fibre layers range from 1 micrometre to 25 micrometre in thickness.

The innermost layer of the cuticle is the
endocuticle. This layer is homogeneous and consists of alternating
dense, less-dense layers. The internal surface is often wavy. The Enoplida, e.g.
Enoplus and Deontostoma have
the most complicated cuticle structure.

At the head end the mesocuticle thickens greatly to form a shock absorber. The sclerotised outer layers form the
head skeleton or endocupola.

The lumens of the stoma (buccal capsule enclosing the buccal cavity), pharynx and posterior intestine are also
lined by cuticle, though this internal cuticle is simpler in structure and consists of epi- and endocuticle only.
Nematodes moult their cuticle 4 times during growth. Both the external and the internal cuticles are shed.

The nematode cuticle is
selectively permeable and may be important in the uptake of certain materials,
especially in Enoplida where pores give the cuticle a high permeability. This semiporous property is thought to be
particularly important in parasitic nematodes. The cuticle is also an effective barrier to many materials. The cuticle
also functions in protection. Those layers of the cuticle that appear clear and homogeneous under the electron
microscope are especially hard and contain
scleroprotein. The lacunar (spongy) layers are effective shock
absorbers, especially at the anterior end of the worm. This protective function is important to nematodes, since
these worms characteristically occupy ‘tight’ interstices in soil, plant tissues and animal guts, where they are
otherwise prone to mechanical damage.

Most nematodes move by undulations of the body or by serpentine crawling. They are not capable of significant
alterations in body shape. Rather their bodies are stiff and turgid. The turgor of the body tissues keeps the cuticle
stretched tight and when the worm flexes its body by muscular contraction the elasticity of the cuticle straightens
the body back out. Thus, essentially we have the muscular-flexing of an elastic cylinder. The compression of the
striations is particularly important in providing a restoring force. The layers of obliquely oriented fibres, present in
many nematodes, presumably function in a similar manner to the spiral fibres in the basement membranes of
Turbellaria and Nemertinea. In nematodes, the spiral cuticular fibres are at 60-75 degrees to the long axis.

Beneath the cuticle lies the epidermis. The epidermis (
hypodermis) is usually cellular, but may be syncitial. The
epidermal cytoplasm extends into the pseudocoel to form middorsal, midventral and midlateral longitudinal cords,
containing the epidermal nuclei in rows. Hemidesmosomes connect the hypodermis to the cuticle. Tonofibrils from
these hemidesmosomes penetrate the cuticle. Interchordal hypodermis is attached to the underlying musculature
by a layer of amorphous intercellular materiel. The total thickness of the interchordal hypodermis is 0.1-0.8
micrometres in free-living forms, but in parasitic forms the subcuticular layer may reach 30 micrometres in thickness.

The hypodermal cells are arranged in 5-12 regular longitudinal rows, and thus there are also 5-12 hypodermal
chords, grouped into lateral, and often also ventral and dorsal rows. The dorso-ventral flexing of the nematode
during locomotion, means that the cell-cell junctions of the lateral hypodermal cells must be particularly strong and
these cells have interdigitating margins that serve to increase the contact area between adjacent cells. The
hypodermis is also an important storage organ, and stores glycogen and fat.
Musculature

Beneath the epidermis is a single muscle layer composed entirely of longitudinal obliquely striated fibres, arranged
in bands. The contractile filaments are limited to the base of broad, flat fibres or to the bases and sides of tall,
narrow fibres. Each muscle fibre has a slender arm extending to the dorsal or ventral nerve cord.

The muscle cells are divided into basal contractile zones. The muscle cells are obliquely striated, with the
contractile filaments at an angle of 14-17 degrees to the z-bands. There are 10-12 actin filaments around each
myosin. The apical cytoplasmic zone contains the cell nucleus, mitochondria and glycogen granules. The
innervation process extends to a nerve trunk or to the nerve ring. Each cell may give out several branched
processes. These form a spatial network in the pseudocoel. Neighbouring muscle cells are connected by electrical
synapses.

In the subclass Chromadoria and some of the Rhabditia, the muscle cells are flat
platymyarian muscle cells: the
z-bands are perpendicular to the base of the cell and at an acute angle to the cell axis. In free-living forms of the
order Enoplida, the muscle cells are primary
coelomyarian muscle cells: the z-bands are parallel to the cell base
in transverse section, and diagonal in longitudinal section. Large parasitic nematodes have secondary
coelomyarian muscle cells. These form ontogenetically from platymyarian muscle cells (hence 'secondary'). In
some Rhabditia (e.g.
Filariina) the contractile zone forms a ring along the whole length of the cell and these muscle
cells are called circomyarian muscle cells. These form from secondary coelomyarian cells. The coelomyarian type
of muscle cell is found in large nematodes and serves to increase the cross-sectional area of the contractile zone.

Polymyarian nematodes have ~10 to ~100 muscle cells, whilst meromyarian nematodes have only 8 muscle cells in
transverse section. Small forms and juveniles tend to be meromyarian. Polymyarianism evolved secondarily in
enlarged parasitic forms, such as
Ascaris, which has 600 muscle cells per transverse section.

The nematode has a constant cell number. In
Caenorhabditis elegans there are 34 muscle cells, 9 marginal cells, 5
glandular cells and 20 nerve cells. Large parasitic forms have larger cells, but the same cell number. In large
nematodes the gland cells contain many nuclei, for example
Ascaris has ~ 10^4 (10 000) nuclei per gland cell.

Locomotion

Many nematodes can swim intermittently for short distances. A few can crawl, undulatory waves of muscular
contraction act against the substrate, aided by the grip provided by the sculptured cuticle. The hydrostatic skeleton
and elastic cuticle are antagonistic to muscle contraction. Nematodes move through soil pores 15-45 micrometres
in diameter, and the pore size for optimum movement is about 1.5 times the worm’s diameter.

In one species, with a ringed cuticle, crawling is earthworm-like. Others may crawl like caterpillars, and others move
like inchworms.

In swimming the dorsal and ventral muscles contract alternately, causing the body to undulate. The dorsal and
ventral muscles are well developed and equally well developed, with equal numbers of muscle cells.
Nutrition

The nematode digestive tract can be divided into an ectodermal foregut, comprising the pharynx and, if present,
the
buccal cavity, an endodermal middle intestine and an ectodermal posterior intestine. Many nematodes
are carnivorous, but some are phytophagous (plant-eating). Many terrestrial nematodes pierce plant root cells and
suck-out the contents. Many are deposit feeders, ingesting the substrate particles and associated microbes.
Similarly, some feed on dead organic matter, such as dung, corpses, etc. and associated microbes.

The mouth of the nematode is usually terminal and either opens into a buccal cavity, or stoma, or directly into the
pharynx. The stoma, or buccal capsule, is tubular and lined with cuticle, which is often strengthened by
rhabdions,
which are ridges, rods or plates, and may bear teeth. For example,
Mononchus papillatus is carnivorous and
toothed. It has a large dorsal tooth opposed by a buccal ridge. One such individual may consume up to 1000 other
nematodes during its 18 week lifespan. It attaches its lips to its prey and makes an incision and pumps out the prey’
s contents with its muscular pharynx. In the Enoplida nematodes the stoma contains 3
cuticular mandibles (jaws
or gnathi). In some nematodes the buccal cavity is divided into 3 regions, an anterior vestibule enclosed by the
lips, a middle protostom, which is the longest region, and a posterior telostom (or glottid apparatus). The mouth is
often surrounded by 3 or 6
labia or lips (see description under Sensory Systems).

Some carnivorous roundworms, and many species that feed on plant cells, have a long hollow spear or
stylet or a
solid odontostyle, housed in the buccal capsule. This stylet can be protruded through the mouth. Enzymes are
released, and in the case of hollow stylets, the digested food is pumped into the worm via the stylet.

The buccal cavity leads into a tubular pharynx or oesophagus. The buccal cavity is itself sometimes embedded in
and surrounded by pharyngeal tissue. The pharyngeal lumen is triradiate in cross-section and lined with cuticle.
The wall is composed of myoepithelium and gland cells. Frequently, there is more than one muscular swelling or
bulb, acting as pumps. Valves are frequently present.

The triradiate symmetry allows the pharynx to be easily and completely closed. The pharynx consists of two
concentric tubes, the cuticular inner lining and the outer basal lamina, with radial muscle fibres in-between.
Contraction of these fibres opens the pharynx, whilst upon relaxation of these muscles, hydrostatic pressure closes
the lumen. Pharyngeal contractions travel from anterior to posterior. The muscular cardia valve separates the
pharynx from the middle intestine.

The second index of de Man gives the ratio, b, of the body length to the pharynx length. In small nematodes b is
smaller (the pharynx is relatively longer). In juveniles b ≈ 2. In small nematodes the undulatory movements of the
worm body reduce the intracavity pressure. This adversely affects pharynx function in small nematodes. To
overcome this, the pharynx is relatively longer (i.e. b is smaller) or a different means of locomotion is employed.

In cross-section the pharynx has three muscular cells (one dorsal and two subventral) and three marginal cells
(one ventral and two subdorsal) opposite to the lumenal radii. Three pharyngeal digestive glands open via ducts
into the pharyngeal lumen. One of these glands is unicellular and dorsal; the other two comprise two cells each
and are subventral. The subventral glands may open via a common duct. There are three pharyngeal nerves, one
dorsal and two subventral. The muscle cells have radial myofilaments and are cross-striated and extend between
the pharynx cuticle and the basal lamina. These muscle cells are only one sarcomere long. The marginal cells are
non-contractile and contain fibrils that connect to the inner cuticle and to the outer basal lamina by
hemidesmosomes.

The pharynx may be simple and cylindrical or may taper anteriorly or it may possess a basal and/or median
muscular swelling with valves, called a bulb, or without valves, called a pseudobulb. In some nematodes, the
pharynx is long, tubular and non-muscular, as in mermithoids. In most Plectida, the pharynx consists of a wider
anterior part, or corpus (divisible into anterior procorpus and posterior metacorpus), a narrow middle part or
isthmus and a posterior bulb.

The pharynx leads into the intestine. A single-celled epithelial layer lines the intestine lumen. A valve at each end
prevents food from being forced out by the hydrostatic pressure of the pseudocoel. The middle intestine has a
fixed cell number in some nematodes; for example it comprises 18 cells in Turbatrix aceti. In this case the cell size
increases during growth, and the cells may become giant (up to 4 mm by 0.5 mm) and multinucleate. In some
nematodes the middle intestine cell number is not fixed and may number ~ 10^2 (100) to 10^6 (1 000 000) cells.
When not fixed in number, these cells can be regenerated and replaced. In this latter case, whole sections of the
intestine can regenerate if a section is removed.

The intestine leads into a short, cuticle-lined rectum (cloaca in males) which connects the intestine to the anus.
The anus is on the midventral line, just in front of the posterior tip of the body.

The pharyngeal glands and intestinal epithelium produce digestive enzymes. Digestion begins extracellularly, but is
completed intracellularly.

Digestion begins in oral cavity and is completed in the middle intestine, where it is both extracellular and
intracellular. The intestine lumen contains a microvilli brushlike border, and may also possess synchronous cilia.
Extracellular digestion occurs both in the lumen and on the surface of the microvilli.
Excretion and Osmoregulation

Protonephridia are absent and some nematodes have no specialised excretory system, but many do possess
gland cells, with or without tubules (glandular ducts), with some excretory function.

In the class Adenophorea, which includes most aquatic species, there is usually one large gland cell or
renette
gland
located ventrally in the pseudocoel near the pharynx. It has a necklike duct that opens ventrally on the
midline as an excretory pore.

In the class Secernentea, which includes many terrestrial species, three long canals are arranged to form an H-
shape. Two of the canals are lateral, located in the lateral longitudinal cords and the third transverse canal
connects these. From this transverse canal a short excretory canal leads to the excretory pore. In some species,
the anterior lateral canals may have disappeared, forming a horseshoe shape. In others the system is asymmetric.

Nervous System

Nematodes have unusual nervous systems in which the muscle cells send out long processes to the central
nervous system, rather than the more familiar case of the central nervous system sending out nerves to the
muscles (as in vertebrates like humans)! In coelenterates, echinoderms, annelids and lancelet, both types of nerve
are found, but only in Aschelminthes like nematodes is the muscular process type of innervation taken to its
extreme development.

The nervous centre is a circumenteric (circumpharyngeal, i.e. surrounding the pharynx)
nerve ring with ganglia
attached dorsally, laterally and ventrally. These ‘ganglia’ are not true ganglia, since they lack neuropil and have no
external sheath covering (only in
Siphonolaimus do they have sheaths) but are simply swellings composed of cell
bodies. The neuropil is in the ring itself, and the whole is functionally equivalent to a ganglion. Hence the swellings
will be referred to here as
pseudoganglia.

In the Rhabdita the lateral pseudoganglia comprise 30-45 nerve cell bodies, the ventral pseudoganglia 16-33
cells, and the dorsal ganglia contain 1-7 cells only, or are absent. The nerve ring gives off
sensory nerves to the
head. These nerves are deep and are adjacent to the body wall. A
subdorsal nerve innervates one labial papilla
and one pair of cephalic setae. A
subventral nerve also innervates one labial papilla and one pair of cephalic
setae. Lateral nerves each innervate one labial papilla, the corresponding lateral cephalic seta and amphid (see
'sensory systems' below).

Nematodes possess 8-12
longitudinal nerve cords. Free-living nematodes have the most complex nervous
systems, such as those of marine nematodes in the order Enoplida. In the main body of the worm there are 10
longitudinal nerve cords: ventral, lateral, paired subdorsal, subventral, ventrolateral and dorsolateral nerves.
These are linked by semi-circular commissures and by individual nerve fibres. The ventral nerve cord (VNC) is the
most developed and connects to the circumpharyngeal nerve ring, as may the other cords. The VNC contains
hypodermal and subcuticular nerve fibres. The other longitudinal nerves contain subcuticular fibres only.

Lateral sensory plexi innervates the somatic sensory setae and the female vulvar region. In the posterior of the
worm the ventral cord bifurcates to form the circumanal commissure and branches of the ventral cord connect to
the lateral cords.

In
Ascaris, the ventral nerve cord contains 55 fibres, the dorsal cord 3 fibres, the submedian nerve cords 4 fibres
each and the lateral nerve cords also contain 4 fibres each. The fibres may be up to 40 micrometres in diameter.

In all nematodes the VNC is the most developed and contains sensory, motor and interneurons. The VNC
connects to the circumpharyngeal nerve ring by a pair of rootlets (subventral nerves). The VNC innervates the
dorsal musculature with motor neurons. It also sends out motor fibres to the dorsal nerve cord (DNC) which go on
to innervate the dorsal muscles. These muscles are responsible for the swimming undulations. They are equally
developed dorsally and ventrally and hence equally well supplied by motor fibres. The DNC gains fibres as it
proceeds posteriorly. In its anterior portion it contains just 3 fibres in
Ascaris, whilst posteriorly it contains 13 fibres.

The DNC and submedian nerves are usually devoid of nerve cell bodies. The submedian nerve cords (SNCs) are
motor, whilst the lateral nerve cords (LNCs) are sensory. The motor nerves (VNC, DNC, SNC) are apparently
cholinergic, as are the amphids, phasmids, genital sensilla and head sensilla. The sensory system, however, is
generally catecholaminergic. In saprobiotic and parasitic nematodes, the sensory systems are reduced and only 4
catecholamine neurons remain. These innervate the cephalic papillae (outer circle), the male genital papillae and
1-2 pairs of lateral sensory neurons.

The pharynx is innervated by its own pair of subventral longitudinal nerves and a dorsal longitudinal nerve,
connected by circular commissures at the base and anterior end of the pharynx (and also by sector commissures).
In
Caenorhabditis the pharynx consists of 34 muscle cells, 9 marginal cells, 9 epithelial cells, 5 glandular cells and
20 neurons (including sensory, motor and interneurons). Longitudinal nerves and a sensory nerve plexus
innervate the middle intestine of parasitic nematodes.
Parascaris has nerve plexi on the walls of the gonoducts.
Bipolar neurons may also be associated with excretory canals.

In the order Enoplida, each worm contains thousands of nerve cells, and the number is variable. In Rhabdita each
worm contains between 150 and 300 neurons, and the number is constant within a species. In small nematodes,
the nervous system may account for 40% of the cells in the worm’s body. It is interesting to note that the cephalic
nerves have hexaradial symmetry, whilst the motor systems have biradial symmetry.

Sensory Systems

The thick nematode cuticle presents a potential barrier to sensory information. To overcome this, nematodes have
receptors with specialised cuticular accessories that allow specific sensory information through the cuticle at
specific sites on the external body surface. These accessory structures serve to filter sensory modalities and in
some cases amplify the sensory signals. The labial papillae and cephalic papillae are cuticular projections, each
containing a nerve fibre that branches from the papillary nerves. Mechanoreceptive setae are present.
Amphids
are blind, pouchlike or tubelike invaginations of the cuticle. These are chemoreceptive and possibly have other
sensory functions. Cervical papillae are situated just behind the anterior region of the worm. The papillae are
conical and often contain a terminal pore or are reduced to mere pores or extended into bristle-like setae.
Mechanosensory setae amplify tactile information detected by a sensory neuron at the base of the seta. Cephalic
slits occur in Enoploidea as a pair of pouch-like cephalic sense organs adjacent to the amphids.

Somatic sense organs are well-developed in free-living forms, as in the subclasses Enoplida and Chromadoria.
These take the form of pores, which are usually chemoreceptive, and of bristle-like setae. In
Paracanthoncus,
pores open at the bottom of cuticular depressions and are arranged in rows over the whole body. Each pore
opens into a canal that opens into a receptor cavity. Each is innervated by 1-2 sensory dendrites. These dendrites
are modified non-motile cilia. Also associated with each sense-organ or sensillum (pl. sensilla) are two modified
hypodermal cells, a glandular sheath cell and a socket cell.

Setae are hollow cuticular processes innervated by 5-6 dendritic branches (the sensory processes of a nerve cell)
and so each is a sensory unit or sensillum (plural: sensilla). The lumen of the sensillum opens to the outside via a
pore in the tip of each seta. In
Deontostoma californicum the somatic setae are cones, each with a pore opening in
its tip. Sensilla are sensors containing at least one sensory nerve cell which is actually a modified epidermal cell,
with a cilium, which is immotile in this case and instead serves as a sensory 'wire' or 'antenna' and is called a
dendrite, which often branches into several dendritic branches. In general, a dendrite is an elongated process and
part of a nerve cell which carries signals towards the main nerve cell body. The sensory dendrites respond to
stimuli, generating electrochemical signals which travel to the sensory cell body, which then relays the signals, via
another elongated process called an axon, to the rest of the nervous system. Insects, which are distantly related to
nematodes, have sensors built around similar sensory units, also called sensilla and the structure of the insect
sensillum is described in more detail on these pages:
insect antennae, insect mechanoreceptors.

The order Trichocephalida is equipped with
bacillary bands: rows of pores and glandular cells of the lateral
hypodermal nerve cords and 4-6 dendritic branches innervate each pore. These branches are embedded in the
glandular sheath cells. These embedded dendrites possibly function as hygroreceptors.  
Caenorhabditis
possesses aporous cuticular papillae each innervated by one electron-dense dendrite.

Sensory dendrites in contact with the outside through a pore are probably chemoreceptive. These are also
associated with glands whose secretions bathe the dendrites. Dendrites with electron-dense tips are thought to be
mechanoreceptive and hygroreceptive. Some sensilla are possibly dual-function chemo-mechanoreceptors, since
they possess dendrites extending to pores and electron-dense dendrites that often terminate in the sensillum base.

In free-living enoplians (O. Enoplida) the
labial papillae and cephalic papillae are conical and each has an
internal canal that opens at the tip via a pore. Shorter sensilla are papillae whilst longer sensilla are setae. Setae
also cover the general body surface, and are irregularly distributed, but with a tendency to concentrate into two
paired sublateral rows. In contrast, the receptors of the anterior end are more precisely arranged. Some examples
of the arrangement of these
anterior sensilla follow.

  1. Pontonema vulgare (O. Enoplida) has 6 labial papillae, 10 cephalic setae and one pair of pocket-shaped
    amphids. The labial papillae and cephalic setae each contain 9-12 dendritic branches, one wide with 80-100
    microtubules per cross-section embedded in electron-dense material, whilst the other dendrites are
    threadlike with ~1 microtubule per cross-section. The amphidial nerve has 17 dendritic branches. One of
    these is widened and vacuolated and contains electron-dense material. The other 16 are threadlike. All end
    at a pore opening at the base of the amphidial cavity. The amphidial cavity in turn opens to the outside by a
    pore or slit. The amphidial cavity is filled with a glandular secretion originating from a gland in the nerve ring,
    far from the head-end.
  2. Xiphinema (O. Dorylaimida) possesses an inner circle of 6 labial papillae and 6 cephalic papillae and an
    outer circle of 4 cephalic papillae. There is also one pair of pocket-shaped amphids.  The inner circle
    papillae have terminal pores and 4 dendritic branches, with one thicker dendrite. The outer circle papillae
    each have 3 dendritic branches, with one thicker dendrite. The amphids contain 14 neurons each (5 of the
    neurons have 2 dendritic branches, giving a total of 19 dendritic branches. One dendrite is short and does
    not reach the amphidial cavity, whilst the rest enter the cavity.
  3. The amphids of Oncholaimus vesicarius (O. Enoplidia) contain 38 dendritic branches, from 3 dendrites. A
    4th dendrite gives rise to 10 branches that enter a depression in the sheath cell and do not reach the
    amphid cavity or the pore in the base of the cavity.
  4. Tobrilus aberrans (O. Enoplidia) have pocket-shaped amphids with 16 dendrites, each with a single branch.
  5. The parasitic Capillaria hepatica (O. Enoplidia) has an inner circle of 6 labial papillae and 6 cephalic
    papillae, an outer circle of 4 cephalic papillae, and one pair of amphids. All these sensilla have pores. Each
    dorsolateral and ventrolateral papilla has two thin dendritic branches that extend to the terminal pore. Each
    lateral labial papilla has 3 dendritic branches, 2 of which are threadlike and do not extend to the pore, while
    the third widens at its distal end and contains electron-dense material. Each lateral cephalic papilla also has
    three dendritic branches. Each amphid has 10 dendritic branches, each ending at a different level inside the
    amphidial cavity or canal.
  6. Sphaerolaimus balticus (SC. Chromodoria) has 6 labial papillae, 10 cephalic setae and 8 pairs of cervical
    setae. Each of these sensilla has one dendritic branch 25 m long, which extends to the pore and has its
    cell body deep in the cephalic tissues. These dendritic branches have electron-dense tips. This species also
    has one pair of circular amphids. Threadlike spiral branches fill the amphidial cavity. One of these dendritic
    branches is thicker and forms a descending spiral.
  7. Paracanthonchus has 6 labial papillae and 10 cephalic setae, similar in morphology to somatic setae, and
    one pair of spiral amphids. Each labial papilla has 5 dendritic branches. Each cephalic seta has 6 short and
    4 long dendrites. The amphids form a spiral trough, filled with secretion, on the cuticle surface. Each amphid
    has 18 dendritic branches, one of which is non-emergent and resides in the sheath cell recess.
  8. Caenorhabditis elegans (a saprobiont) has 6 labial papillae, 6 cephalic papillae, 4 outer cephalic papillae
    and one pair of porelike amphids. Each labial papilla has a small terminal pore and 2 dendritic branches,
    one of which extends to the pore, the other, which is electron-dense, ends in the base of the papilla. The
    subdorsal and subventral papillae of the inner and outer circles pair to form double submedial papillae. In
    each of these, two dendritic branches with electron-dense tips extend into the cuticle, connected by a strand
    of dark material to the outer cuticle layer. Each amphid contains 10 dendritic branches (8 dendrites, 2 with 2
    branches) and three more dendritic branches enclosed within the sheath cell and another dendrite,
    equipped with about 50 microvilli, is enclosed in the sheath cell, which is situated in lateral pseudoganglion
    at the level of the nerve ring. Thus each amphid contains a total of 12 dendrites.
  9. Nippostrongylus brasiliensis (an animal parasite, SC. Rhabdita) has 6 labial papillae, 4 double submedian
    cephalic papillae (the lateral cephalic papillae are reduced) and one pair of porelike amphids. Each labial
    papilla contains one dendritic branch that terminates in an electron-dense tip in the cuticle, and is
    connected to the surface layers of the cuticle by a dark strand. The submedian cephalic papillae each
    contain 2 dendritic branches with electron-dense termini. One of these is bent parallel to the outer cuticular
    surface. Each amphid contains 13 dendritic branches extending to the pore, and 2 unbranched dendrites,
    equipped with microvilli, that terminate in the sheath cell.
  10. Heterakis gallinarum possesses three secondary labia around the mouth, which bear 6 labial papillae. It also
    has 4 double submedian cephalic papillae, formed from the fusion of the inner and outer circle cephalic
    papillae, and there is one pair of amphids. The two pairs of lateral papillae are reduced and in their place
    are cervical papillae that have moved anteriorly and are just posterior to the secondary labia. The labial
    papillae are pointed and each has a single dendritic branch in the cuticle at its base. Each submedian
    papilla has one pair of dendritic branches that terminate in its cuticle. These have widened distal ends and
    are arranged at right angles to each other. The lateral papillae are cuticular processes each with a pore in
    its tip and containing two dendritic branches, one extending to the pore, the other is electron-dense and
    ends at the base of the papilla. Each amphid contains 13 dendritic branches extending to the amphidial pore
    and two additional branches end inside a hypodermal cell.
  11. In the order Tylenchida, the inner cephalic papillae are generally not connected to the external environment.
    Each contains a single papilla terminating in the cuticle. The outer circle papillae usually have no openings.
    The amphids contains 7 dendritic branches extending to the amphidial pore, 5 dendritic branches
    terminating in the sheath cell and another dendritic branch, equipped with about 200 microvilli, also
    terminates in the sheath cell.

Labial papillae may have pores that connect to the inside of the oral cavity, as in
Radopholus, Rotylenchus and
Macroposthonia.

Thus, the receptors of the anterior end are precisely arranged in various patterns. All these patterns, however, are
possibly variations on the same probable ancestral plan. This consists of rings of 6 setae. Thus, one would have
an inner circle of 6 labial setae, and two outer circles of 6 cephalic setae (6 + 6 + 6 arrangement). The lateral
setae of the third circle are though to have developed into the amphids, leaving a 6 + 6 + 4 arrangement. The 4
setae remaining in the third ring often united with the second ring to give the 6 + 10 arrangement of two circles
commonly seen, as in the Enoplida. In some nematodes, these have been supplemented by cervical setae uniting
with the cephalic setae. Thus, we have a concentration of receptors at the anterior end. Furthermore, the most
anterior setae often shorten into papillae. This is possibly a mechanical adaptation, since the anterior setae are
likely to be involved in forceful collisions with obstacles. These changes were also accompanied by changes in the
mouth, from a triradiate mouth following the contour of the triangular pharynx and lacking labia, to mouths with
three and then sometimes six labia. The anterior sense organs may be reduced in parasitic forms.

Although the oral aperture has tri- or hexaradiate symmetry, the labial papillae have hexaradiate symmetry, the
amphids are biradially arranged and cephalic setae are biradially arranged in the Enoplida (2 pairs subdorsal, 2
pairs subventral and 1 pair lateral), the overall body symmetry is bilateral about the sagittal plane.

Ocelli are found either side of the pharynx in some aquatic nematodes. Stretch receptors in the epidermal cords
are involved in regulation of locomotor movements.

Metanemes are proprioceptors situated in the lateral hypodermal chords. Each metaneme consists of a primary
sensory cell whose ciliary dendrite is inserted into the receptor cavity of a secondary sensory neuronal dendrite.
The wall of the receptor cavity contains granules and is in synaptic contact with the ciliary dendrite of the primary
cell. These receptors apparently detect the ventrodorsal bending of the nematode body during locomotion.

The sensilla all have glandular cells associated with them. These produce secretions that bathe the dendrites and
are necessary for sensory function. This arrangement has lead to the evolution of organs with dual sensory and
glandular functions.
Adhesive setae occur in the order Desmoscolecida and the family Draconematidae. The
adhesive secretion is emitted through the terminal pore of the seta and is employed in the caterpillar-like crawling
mode of locomotion of these nematodes.

In male nematodes there is a midventral preanal row of 10-20
supplementary organs that have dual sensory
and adhesive functions. They serve to attach the male to the female during copulation. Male nematodes of the
subclass Rhabditia also have paired pre- and postanal papillae. In Aphelenchoides each papilla has a terminal
pore with a single chemoreceptive dendrite terminating just beneath this pore and a basal mechanoreceptive
dendrite with an electron-dense tip. In Dipetalonema, each papilla contains a single dendrite with an expanded
mushroom-shaped electron-dense tip that is connected to the outside via a narrow canal.

In nematodes of subclass Rhabditia, there may be a
copulatory bursa supported by ribs. In Pelodera there are
dendritic branches that terminate beneath pores at the end of each rib, and are presumably chemoreceptors.

In male nematodes the spicules are hollow protuberances with distal pores, and contain sensory dendrites. These
are presumably tangoreceptors, though in
Tylenchulus semipenetrans they also have mechanoreceptive dendrites
and are dual function receptors. In
Pelodera, the spicules are protruded only after the final attachment of the male
to the female. The male gubernaculum is also innervated by sensory dendrites, that are enclosed and not in
contact with the external medium.

A pair of unicellular glands, called
phasmids, open separately on either side of the tail in some nematodes
(Secernentea). These are most well developed in parasitic nematodes and may function as glandulosensory
chemoreceptors. They are also often better developed in female nematodes.
Glandular Systems

The cervical gland or renette opens via a cuticularised canal to the ventral anterior excretory pore, anterior or
posterior to the nerve ring. This gland has a neurosecretory function, in some nematodes, releasing an enzyme
that dissolves the old cuticle during molting. In general, though, the cervical gland is thought to have an excretory
function.

Free-living Enoplia and Chromadoria have
caudal glands that form a complex organ: the spinneret. This
secretes a sticky substance that allows the nematode to adhere to the substrate. It may be used to stick the
nematode to a particle of detritus that is subsequently ingested.

Crytobiosis (anabiosis)

Cryptobiosis is a state of dormancy, in which many species can survive dry conditions for several years. Inactivity
is accompanied by water loss and a very low metabolic rate. These dormant states can also tolerate low
temperatures.

Reproduction

Most nematodes are dioecious. The males are often smaller, and have a hooked tail. The gonads are tubular. The
germ cells usually arise from a single large terminal cell located at the distal end. The germ cells gradually pass
through the gonad, maturing as they do so.

The male system comprises 1-2
testes, which pass into the long sperm duct, which widens to form a long
seminal vesicle. A muscular ejaculatory duct connects the seminal vesicle to the rectum / cloaca. The
ejaculatory duct is lined with
prostatic glands, which secrete an adhesive material to aid copulation. The wall of
the cloaca is evaginated to form two pouches, which join before they enter the cloacal chamber. Each pouch
contains a pointed curved
spicule. Special muscles can cause the spicules to protrude from the anus or vent. In
many nematodes the dorsal walls (and sometimes ventral and lateral walls) of the pouches bear special, cuticular
pieces forming the
gubernaculum that guides the spicules through the cloacal chamber.

The female system consists of 1-2
ovaries, usually oriented in opposite directions. The ovary gradually extends
into a tubular
oviduct and into a widened, elongated uterus. Each of the two uteri opens into the vagina, which
opens to the outside via the
gonopore located on the midventral line in the middle of the body. The upper end of
the uterus usually functions as a seminal receptacle.

The females secrete a pheromone that attracts males. The curved posterior of the male nematode is usually
coiled around the body of the female in the region of the genital pores. The male extrudes its copulatory spicules,
which are used to hold open the female gonopore during sperm transmission. The amoeboid sperm migrate to the
upper end of the uterus, where fertilisation occurs.

Some terrestrial nematodes, e.g. rhabditoids, are hermaphroditic and eggs develop after sperm develop in the
same gonad (an
ovatestis). Self-fertilisation then occurs. Parthenogenesis also occurs in some nematodes.

Marine species produce about 50 eggs, laid in clusters. Terrestrial forms produce several hundred eggs. Viviparity
occurs in many parasitic nematodes and also in some freeliving species, as in the vinegar eel.

Development

After fertilization the oocyte cytoplasmic membrane exfoliates and becomes the vitelline membrane. A new
cytoplasmic membrane is formed beneath this vitelline membrane. In the uterus, the egg envelope is formed with a
chitinous shell. Free-living nematodes deposit 1-50 ova at a time, stuck together in chains or clusters. These eggs
may be stuck to the cuticle of the adult. Parasitic forms produce thousands of ova.

Aberrant cleavage (i.e. cleavage that does not adhere to the usual radial or spiral cleavage patterns) gives rise
to a blastula and then a gastrula. The blastula is determinate, in that cell fate is determined at this stage. The
blastopore forms the mouth and anus. The worms hatch as juveniles. Subsequent development is direct and
maturity is generally reached after the fourth
molt (shedding of the cuticle).

Ecology

The nematode body-plan is an extremely successful one and there are some 25 000 described species.
Nematodes occur in all habitats, from arid deserts to the bottoms of lakes, rivers and at a range of depths in the
oceans and a range of temperatures, from hot springs to polar seas. Free-living forms are particularly suited to
intersticial meiofaunal habitats, between sediment particles, and their narrow form and tough cuticle enable to
penetrate small spaces without being easily crushed or abraded. This life-style pre-adapted nematodes to a
parasitic mode of life, in the tight tissue spaces of plants and animals.

Free-living marine herbivorous nematodes eat mostly diatoms, whilst fresh-water herbivorous forms eat green
algae and cyanobacteria. Saprophagous forms eat detritus, dead and decaying matter and the bacteria and fungi
found therein. Carnivorous nematodes eat mostly rotifers, tardigrades, annelids and other nematodes. Many
nematodes are zooparasites and many are phytoparasites.

Classification

O = order:

O. Enoploidea: Free-living, mostly marine.
O. Dorylaimoidea: common in soil and fresh-water.
O. Mermithoidea: juvenile stages parasitic in terrestrial or fresh-water invertebrates, usually insects, whilst the
adults are free-living in soil or water. Up to 50 cm long and filiform.
O. Chromadoroidea: mostly marine with spiral amphids and usually a ringed or punctated cuticle.
O. Araeolaimoidea: fresh-water or terrestrial with 4 conspicuous cephalic bristles.
O. Monhysteroidea: aquatic or terrestrial, but mostly marine.
O. Desmoscolecoidea: short, plump, marine nematodes with heavily ringed and more or less bristled bodies and
hemispherical amphids and a demarcated armoured head with 4 bristles.
O. Rhabditoidea or Anguilloidea: papillate cephalic sensilla and 1-2 pharyngeal bulbs. Semiparasitic or epizoic, or
parasites / facultative parasites in vertebrates and invertebrates, or phytoparasites – often gall-forming, or
terrestrial and free-living, inc. vinegar eels and nematodes found in felt beer mats.
O. Rhabdiasoidea: vertebrate parasites with complicated life-cycles.
O. Oxyuroidea: zooparasites, esp. of vertebrates, with a one-host life-cycle.
O. Ascaroidea: vertebrate intestinal parasites.
O. Strongyloidea: bursate nematodes. The male has a conspicuous, expanded bursa supported by muscular rays.
Intestinal parasites of herbivorous mammals and human hookworms, which penetrate the skin.
O. Spiruroidea: parasites of mammals, inc. eye worms (Thelazia, Oxyspirura), birds and fish. May have
invertebrate intermediate hosts.
O. Dracunculoidea: filiform, parasitic in connective tissue or coelom of vertebrates. Have an intermediate host
(typically a copepod).
O. Filarioidea: filiform, males much smaller than females. Endoparasites of vertebrates, transmitted by blood-
sucking insects.
O. Trichuroidea (Trichinelloidea): inc. the whipworms, mammalian intestinal parasites and parasites of the urinary
bladder and air passages of mammals and endoparasites of mammals.
O.Dioctophymoidea: endoparasites of mammals, e.g. the kidney worm, which infects the kidneys (usually the right
kidney) and abdominal cavity of dogs and is up to 1 m long.

(In some classification systems, ‘oi’ is replaced by ‘i’ and ‘ea’ by ‘a’).
Nematodes (Round Worms)
Above: the nematode intestine is designed to withstand mechanical stresses
due to the passage of food and the flexing of the nematode as it swims. To
resist this buffeting, the cells making up the intestine (at least in
Caenorhabditis elegans in which this has been studied) contain
intermediate filaments (IFs) which are intracellular protein fibres and a
component of the cytoskeleton. IFs occur in vertebrates, for example, keratin
gives the cells of skin, hair, horn and nails their toughness. Invertebrates
may lack IFs altogether or have different types. In the nematode intestine
this fibres may be part of the terminal web of the epithelial cells - a 2D net of
actin fibres and IFs that spans the cytoplasm at the apical end of the cell
(that end near to the intestinal lumen). Actin fibres form bundles in the
microvilli, microscopic finger-like projections of the cells into the lumen, which
serve to increase the membrane surface area (for absorption and
secretion), and the roots of these bundles connect to actin and IFs in the
terminal web. The IFs are held under tension as they join to apical junctions
between the cells. [Based on diagrams in Carberry
et al. 2009).
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