The visceral system (pharyngeal)

Published: Last Edited:

This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.

Body symmetry

the nematode body organisation combines organs arranged according to a bilateral symmetry, and a radial symmetry,the latter characterizes almost exclusively the anterior body region

Bilateral symmetry

Submedian axes = 4 equal quadrants

dorsal quadrant(= 90°)

2 lateral quadrants

ventral quadrant

devided quadrants = regions referred to with «sub

Triradial symmetry

Dorsal «sector»

2 ventrosublateral sectors

RADIAL SYMMETRY: head region

Dissymmetry of lipregion in Bunonema: the right ones more developed than the left

Body cuticle… all nematodes posses the same cuticle?

with transverse striae

smooth cuticle


Cuticle with longitudinal striae and ridges

Lateral differentiation of the body cuticle

Perineal pattern in Meloidogyne

bursa male Scutellonema

Caudal alae or bursa


Cuticle with bands of secretion and foreign particles

Body wall

Body cuticle:acellular, layered, proteinous, secreted by the epithelial cells covering the body (epidermis, seam cells*, interfacial cells)

Different cuticle structure in different developmental stages of a single species

Parasitic females of Fergusobiasp. of fly:

-no cuticle

-stylet lost

-pharynx and intestine degenerated

Lateral fieldwith different cuticle ultrastructure compared to dorsal and ventral sides of the body

1 = epicuticle 2 = cortical zone 3 = median zone 4 = basal zone

95% of all nematodes have a cylindrical body shape

-No locomotion by kinocilia; no circular muscles

Radialstriae of corticalzone and basalzone homologous: NO

Basal radial striae: homologouswithspiral fibre layers: NO

Basal radial striae:

1. structural constraints

-e.g. interrupted in obese females-e.g. Trichinella spiralisJ1: disappear during growth (L increases >6 times) but reappear in late

2. Related to environmental stress

(J1-J3 often free-living; dauer)

3. Are involved in elongation of embryo

Surface coat

Functions:facilitate movement; influence cuticle permeability; resistance to immune attack; protection from predators, parasites; in plant parasites the surface coat components may induce a defense response to damage of the plant cell

The body cuticle covers the entire body surface and penetrates inside through various openings, pores.

Formation of cuticle:

1.The structural proteins(collagen, cuticlin): secreted by epidermis

2.Non-structural proteins(e.g. surface coat) are secreted by:

-epidermis(filarial nematodes e.g.)

-pharyngeal gland cells(e.g. in Toxacara canis)

-caudal glands(microbial garden in desmodorids)

-amphid and phasmid support cells,secretory-excretory cell,

Functionsof the body cuticle:

•a protective barrier(withstandextremetemperatures, dehydratione.g. dauer juvenile, ensheathed J3 and important source of antigens)

semipermeableand metabolicactive (continuousproduction and export of surface components e.g. fromepidermis; presenceof pores)

Permitsmotility(actas externalskeleton)


1.Old cuticle is completely shed

2. Cuticle partially resorbed

Changes during the moulting process:

apolysis: inactive period: cuticle becomes loose from tip of head, in buccal cavity and around tail

formation of a new cuticle and ecdysis:-change in ultrastructure of epidermis: intense biosynthetic activity (numerous mitochondria, ribosomes, endoplasmatic reticulum); -separation of cuticle from surface of epidermis: formation of fine fibrillar material in between and epidermal folds (plicae)

At each juvenile stage a new cuticle is generated-Cuticles of different stages differ in their protein expression, number of layers, relative thickness and composition

  1. Epidermal specialisations and their functions

-Epidermal glands:

-secretory function(e.g. formation of surface coat of cuticle),

-role in locomotion,

-absorption (?osmoregulatory function: no) (bacillarybands),

-association with ectosymbiotic bacteria coating the body cuticle: adhesion, host/symbiont recognition (Stilbonematinae)

-Caudal glands: role in locomotion, attachment/ release

-Ventral gland(s)of secretory-excretory system: secretory/ excretory function

  1. Somatic musculature

a single layerof obliquely orientated longitudinalmuscles

No circular body muscles

contractileand non-contractileparts; obliquely striated

A musclecell: (1) contractilepart, (2) non-contractilepart, (3) arm or innervation process

Meromyarian=up to 5

Polymyarian=6 or more

No Z-discs as in Vertebrates but dense bodies

Mechanical significance of obliquely striated muscles in nematodes

Attachment of every sarcomere to basal lamina via dense bodies. Force of contraction is transmitted laterally to cuticle rather than longitudinally to the muscle ends

Obiquelyarrangedsarcomeresdistributethe force application sites of the sarcomere more evenlyover the basal lamina and cuticle, resultingin smoothbending

The body cavity may contain:


-membranes with associated (large) cells or strands suspending organs in large specimens

-crystalloids: especially in freshwater nemas, e.g some Tobrilidae, also in mermithids; contain glycoprotein and hydrogen sulphide (also in somatic muscles of Tobrilus) ? Storage of waste products

-(pseudo)coelomocytes variable in number and position according to the group

-haemoglobinstrands of connective

Funtion of pseudocoeloom

-Excretory functionSphaerolaimus: degradation of urate in organels of pseudocoelomocytes, involved in the metabolism of body fluid

-Secretion and release of bio-active molecules(e.g. immune evasion , e.g. local paralyses of intestine by Parascaris)

Turgor-pressure system(locomotion, =skeleton)

-Osmoregulatory function: difference in tolerance of osmotic stress

Cheilostom(e)= stoma s.s

buccal cavity or stoma s. l.= cheilostome + pharyngostome

Pharyngostom(e)(= oesopha(go)stom(e) of Inglis, 1966

lined with pharyngeal cuticle = 3-radial symmetrical

-cuticle secreted by (1) pharyngeal ectoderm (anteriorly ev. differentiated arcade tissue)and (2) by myoepithelial cells

-shape varies in relation to the feeding type

-= anterior modified feeding apparatus

Wieser (1953) developed a classification of feeding types of aquatic nematodes based on shape and structure of Buccal cavity

Buccal cavity without teeth:

-A. selective deposit feeders (bacteria, flagellates, detritus, unicellular algae)

-B. non-selective deposit feeders (diatoms chlorophyta, flagellates, detritus

2. Buccal cavity with teeth:

-A. selective epistratum feeders (bacteria,

flagellates, rotifers, diatoms, tardigrates, algae, detritus)

-B. carnivores (nemas, rotifers, tardigrates Bryozoa, algae, detritus)


-Stoma tubular or barrel-shaped, without teeth or stylet

-Pharynx variable, never flask-shaped

Predacious nematodes

-Stoma barrel-shaped with large tooth (teeth) or stoma narrows with wide hollow stylet

-Pharynx variable: flask-shaped, cylindrical or with mid-bulb


-Stoma variable, with small tooth (teeth) or narrow with small, short hollow stylet

Pharynx variable

Stylet types

1. a dorylaimid stylet or odontostyle

2. a tylenchid stylet or stomatostyle

3. Stylet-shaped tooth or onchiostyle


(Dorylaimina)derived from a grooved tooth implanted on a ventrosublateral sector of the anterior stomodaeum

Odontostyle secreted by a long cell (cell body in in vsl sector of pharynx, cell tip in vsl sector odontophore level

Odontophore, guiding ring, guiding sheathand lining pharynx: derived from pharyngeal tissue

Pharynx (= “oesophagus”)

- variable in shape in relation to functional

demands (less variable than pharyngostom

- undivided pharynx

- subdivided pharynx

Diversity of “feeding

structures” is linked to

feeding biology

1. Bacterial feeders and

feeders on protista=valvular apparatus

2. Species consuming liquid food such as plant

parasites have a median bulb derived from the posterior

part of the corpus i.e. the metacarpus

Inside the pharynx exists a visceral nervous system, with its

own neurons, receptors

Cardia or pharyngo-intestinal junction

cardia = posteriormost part of STOMODEUM

a “valve” that prevents regurgitation of food

Cardia: an important taxonomic feature for Mononchida

Mononchina=Non-tuberculate funnel-shaped cardia valve

Anatonchina=Tuberculate funnel-shaped cardia valve

Pharyngo-intestinal junction in tylenchids Valve= 2cells

Cardia in Hexatylus:

• no specialized valve cells;

• cardia consists of extensions of two pharyngeal cells

Cardia of Caenorhabditis elegans: 6 cells organised in 3 sets

Cardia in Strongylus vulgaris: 3-lobbed

The mesenteron or intestine or midgut

Possible differentiations:

• anteriorly: ventricular region

• posteriorly: prerectum

In insect parasitic taxa: infective

juveniles e.g. Steinernema with Xenorhabdus

Heterorhabditis with Photorhabdus

The intestine is modified

into a storage organ or trophosome (Mermithidae)


- single layered epithelium

- basal membrane

- lumen with microvilli

-epithelium mostly cellular

but in some zooparasites

syncytial (Haemonchus) orpartial syncytial ( Ancyclostoma

The mesenteron or intestine differs in number of cells in different taxa

- oligocytous = < 128 cells (10 cleavages)

- polycytous = from 256 (11 cleavages) to 8192 cells

(=17 cleavages)

- myriocytous = > 9182 cells

e.g. 20 cells in Caenorhabditis elegans = oligocytous

Intestinal microvilli in tylenchids: evolution from Tylenchidae (present) to Heteroderidae (absent)

Aphelenchoides/ Ditylenchus

Primitive forms with microvilli + terminal web (Aphelenchidae,Aphelenchoididae, Tylenchidae,Anguinidae, Neotylenchidae)

Advanced forms without

(Pratylenchidae, Heteroderidae e.g).

no microvilli: intestine =storage organ


juvenile parasite

Dauer: lumen shrunken, number of microvilli reduced

In Meloidogyne: secretion of rectal glands

produce gelatinous matrix for egg mass

Defecation motor process

Posterior body contraction/anterior

body contraction/enteric muscles


The secretory-excretory system

  1. the glandular type;

-single ventral gland cell or renette cell

- gland duct (terminal duct)

- a ventral S-E pore

(2)tubulo-glandular system

= H-shaped: 5 cells

Functions of the secretory-excretory system

(1) osmoregulation (valve system; pulsating

(2) secretory functions: specialisations

(a) gelatinous matrix for egg masses (Tylenchulus)

gelatinous matrix

(b) secretion of tube (marine nemas

(c) Secretion of glycoprotein of surface coat (labelled

fluorescent lectin staining)

(d) production of enzymes and anti-enzymes (animal


(e) extra-corporal digestion? (esterase, cholinesterase in

Nippostrongylus, anterior position of S-E-pore)

(f) secretion of fluid: start of moulting process

(g) role in locomotion (temporary attachment; material on

surface coat)

Nervous system

  1. The central system

Cell bodies (ganglia)

6 papillary ganglia

dorsal ganglion

2 lateral ganglia

ventral ganglion

retrovesicular ganglion

+ ganglia in tail region: 1 pre-anal ganglion, 2 lumbar ganglia, 1 dorsal rectal ganglion

Nerve ring

- Processes of the anterior receptors (papillary nerves)

- Processes of the interneurons and motor neurons

2. The peripheral system

-cephalic sensillar nerves=6 cephalic sensillar nerves: 2 subdorsal, 2 lateral, 2 subventral: dendrites end in labial and cephalic sensilla (LV)

ventral nerve = largest nerve, located in ventral chord; a sensorymotor nerve; its motor neurons innervate ventral and dorsal somatic muscles

- dorsal nerve without ganglia; essentially a motor nerve + sensory part anteriorly

- two lateral nerves: run from lateral ganglia to lumbar ganglia;mainly sensory

- four submedian nerves : 2 laterodorsal, 2 lateroventral between epidermis and somatic muscles; sensory

- amphidial nerves

- peripheral nerve net=latticework of nerves connecting papillae and setae

- somatic nerves=acetylcholine is the most common neurotransmittor

- genital papillary nerves (from lateroventral and ventral nerves)

- caudal sensilla (innervation by lateroventral nerves)

- deirids = cervical papillae (cell bodies of neurons in lateral ganglia)

3.The visceral system (pharyngeal)

Inside the pharynx exists a visceral nervous system, with its own neurons and receptors

Interneurons (8), motor neurons (7), neurosecretory motor neurons (2), motor-interneurons (1) and marginal cells

Sense organs or sensilla

  1. neural elements: bipolar neurons
  2. non-neural elements: - the socket cell and - the sheath cell (or gland cell)

anterior sensilla: (primitive pattern)

2 circlets of 6 labial sensilla

- 4 cephalic sensilla in a third circlet

Always anterior migration of sensillae in ontogeny

Cephalic sensilla mostly shorter than external Labial sensilla

Post-embryonic development

Cephalic setae have moved forward in adult compared to juveniles

Cervical setae are more anteriorly in position in adult

  1. Segmented setae are primitive (plesiomorph)

6+6+4: all sensilla segmented

6+6+4: only external labial sensilla segmented

All sensilla in the same circle have the same length (is primitive)

Setiform sensilla are more primitive than papilliform sensilla

Labial sensilla (l.s.) and cephalic sensilla (c.s.) are of different origin; somatic sensilla and c.s.

contain catecholamines (= neurotransmittor); l.s. do not.

Somatic sense organs

somatic setae

- body pores

- deirids (= cervical papillae), post deirids

- genital sense organs

- phasmids

- metanemes (enoplids)

- ocelli or eyes

- touch receptors

scutellum = phasmid with large plug

-Phylogenetic assessment:

(1)modified caudal glands? NO

(2) similar to deirids? Yes, is a combination of a lat.epid. cell and a sensillum

= chemoreceptors when a pore is present

Metanemes (meta = one after another; nema = thread) lie laterally in

epidermis in ENOPLIA

Nematode ocelli components

Nervous system and photoreceptor dendrite

Accessory structures:

􀁺 block light from certain directions e.g. shadowing


􀁺 Focus light e.g. lens

sensory cell or photoreceptor of the lamellar type

Photoreceptors of the ciliary type: modified cilia at end of

amphidial dendrite

Reproductive system

Reproduction is sexual (no vegetatief reproduction):

1. Gonochorism (Gk. gonos = offspring; chorismos = separation) = presence of males and females. Such species reproduce by amphimixis (Gk. amphi = both; mixis =mingling); amphimictic reproduction implies copulation, internal fertilization

2. Uniparental reproduction or autotoky:

- parthenogenesis (Gk. parthenos = virgin; genesis = descent)

= development without fertilization

- automixis (Gk. auto = self): bisexual individuals (proterandrous hermaphrodites)

Female reproductive system

amphidelphic =uteri opposed

didelphic=referring to two complete genital branches, two uteri

vulva at mid-body


telogonic development: gamete formation at the tip and, progressive

stage of development along the length of the gonad

hologonic development: germinal zone along the entire length of the gonad; development radially across the tubule, matured gametes move through a more central lumen

The demanian system in Oncholaimidae

= a tubular system that connects the uterus by means

of a duct to a specialised part of the intestinal wall,

known as the osmosium

Function of demanian system

(1) a receptaculum seminis

(2) a secretory function: formation of elastic girdle in pre-anal

region; sex attractant; protection of deposited eggs; present in

absence of males

(3) in a more complex function (cf Oncholaimus) it provides

the natural way of sperm for fertilization;

(4) increase of body pressure in “low pressure” species (sperm

injection, glandular epidermal tissues in posterior body region

of female) and thus facilitates egg deposition;

(5) sperm as additional food and osmosium as safety valve

The male reproductive system

Diorchic condition

(Gk. dis = twice, orchis = testis) = 2 testes that open into a common vas deferens

Monorchic condition:

only one testis, usually reduction of posterior testis (Secernentea

Egg shell: 3 basic layers (may vary between 1 and 5 layers

-(1) outer vitelline layer (derived from oolemma): first layer formed after sperm penetration

-(2) middle chitinous layer

-(3) inner lipid layer

General characteristics of nematode sperm

  1. Shape/size: variable form from round to elongate and variable size
  2. absence of cilia or flagella
  3. presence of unusual membranous organelles (MO)in mature spermatozoa (MO = membrane bound vesicle)
  4. development of pseudopodia, at least in uterus and amoeboid motility
  5. presence of a special major-sperm-protein

6. Absence of a nuclear envelope (except in Enoplida)

7 .modified centriols with triplets replaced by singlets

Hermaphroditism has evolved at least 9 times from gonochoristic ancestors


Polarity egg may be determined by entry of sperm

Anterio-posterior axis determined by fertilization and sperm entry

Cortical cytoplasmic streaming anteriorly

Inner cytoplasmic streaming posteriorly

In Acrobeloides, the sperm cell does not induce the polarity


-Juvenile hatches from egg laid by the female, except in ovoviviparous genera where juveniles hatch within the uterus of the female and subsequently emerge; -Eggs laid individually; each egg with a single juvenile-Juveniles hatch as J1or J2 (most plant-parasitic nematodes)

or J3 in some animal parasites

Ovoviviparitycan be induced in normally reproducing females under the influence of polutants e.g. sulfur dioxide

Eggs of most species laid individually, exceptions are cyst nematodes and root-knot nematodes with several hundreds of eggs in a cyst or in the gelatinaous matrix of root-knot nematode


in response to stimuli of suitable environmental conditions(oxygen availability, t°, soil moisture levels, change in photoperiodicity, absence of physical barriers (e.g. diapause or time set preventing physiological change)

-host-derived hatching stimulie.g. stimuli by root diffusate from host plants

-Other stimulants: microbial hatching factors, chemicals (biocontrol: suicidal hatch)

Hatching process-changes in eggshell: changes in lipid layer (permeability);role of components of perivitelline fluid (trehalose: reduction of water, enzyme activity), release of enzymes (chitinase, collagenase) by juvenile in hatching process-activation of the juvenile:start of pharyngeal pumping, change of amphid structure, change in activity of dorsal pharyngeal gland-eclosion:--widespread exploration--local exploration--cutting cycle

At hatching: S-E-system, pharynx and anterior sensory system are completely formed

Postembryonic development-characterized by four moulting processes -only three juvenile stage e.g. in some longidorid species, Pristionchus

-moults are necessary for growth, but growth occurs to a lesser extend also in between moults and in the adult (especially in animal parasitic nematodes e.g. in Ascaris)

-during moult = replacement of body cuticle, adaptation to life cycle/host(s)

Survival strategies

capacity adaptations: nematode grows and reproduce under harsh condidition e.g. Plectus antacticus: egg laying at 5°C

resistance adaptations:-

-high reproductive capacity, high fecundity of parasites e.g. Ascaris, Globodera-

-survival stages: egg, cyst, dauer juvenile-

-infective juvenile J3

Dauer (= German for to last):e.g. in Caenorhabditis elegans.In response to increase in pheromone production due to crowding or decrease of food signal from bacteria (temperature dependant

Dauer: cuticle thickened + radial striae; mouth closed (non-feeding), pharynx, isthmus and end bulb constricted, lumen intestine shrunken; S-E glands inactive; modification of anterior sensorial organs

phoresis (carrying another organism without parasitizing) is not uncommon

Phoresis: external on the leg bases of horseshoe crabs (Halicephalobus limulus

Others live inside tubes or burrows of polychaete worms (H, e.g. Enoplus communis);in gill chambers of Crustacea (C, monhysterids, Theristus), inside sponges (F, enoplids: Deontostoma),in the holdfast of algae (D, chromadorids, Acanthonchus), in the byssal threads of Bivalvia)

Hermaphroditism has evolved at least 9 times from gonochoristic ancestors