Nematoda

Outlines

PHYLUM NEMATODA

CLASS Adenophorea: Amphids but no phasmids. Most free-living. Simple, not cuticularized excretory system.

SUBCLASS Enoplia
- ORDER Enoplida
- ORDER Mermithida
SUBCLASS Chromadoria

CLASS Secernentia: Amphids and phasmids present. Excretory system usually complex, cuticularized. Mostly parasitic.

SUBCLASS Diplogastria
- ORDER Aphelenchida
- ORDER Tylenchida
SUBCLASS Rhabditia: Small, mainly free living.
- ORDER Rhabditida
- ORDER Ascaridata: Ascaris, Enterobius
- ORDER Strongylida: Ancylostoma, Strongylus.
SUBCLASS Spiruria
- ORDER Spirurata: Wuchereria, Onchocerca, Gnathostoma

PHYLUM NEMATODA

Body Plan

Triploblastic but pseudocoelomate. Body long and filamentous. Unsegmented. Cuticle well developed and complexly layered; secreted by epidermis. Parasitic nematodes have dense fibrous layer under cuticle, not found in most free-living forms. Epidermis cellular or syncytial, often protrudes into coelom as dorsal and ventral cords (housing nerves) and lateral cords (housing nerves and, if present, excretory system). Synapomorphies include muscle cells with filaments extending to dorsal and ventral nerve cords, and special sense organs called phasmids and amphids

Locomotion

Thick layer of obliquely arranged muscle fibers just under epidermis, connected to nerve cords by "muscle arems" (unique to nematodes). Fluid in coelom acts as hydrostatic skeleton. Absence of circular muscles precludes peristaltic movement, so motion is always undulatory; nematodes are poor swimmers and usually require contact with a substrate to move.

Feeding and Digestion

Great variety. Free leiving forms may be carnivorous,detritivouous (some feed on fungi in detritus), scavengers, herbivorous, and bacterivorous. Parasites consume host tissues or body fluids. Digestive tract is complete, with specialized regions: anterior mouth, muscular and glandular pharynx (esophagus), long, straight midgut (intestine), rectum, and ventral anus. Phanyngeal glands (and possibly midgut cells) secrete digestive enzymes, midgut cells have microvilli for absorption.

Sense Organs

Wide variety of setae and papillae which are tactile (mechanoreceptors). Amphids and phaasmids are chemoreceptors unique to nematodes. Some have proprioceptors in lateral epidermal cords to monitor bending. Some free-living forms have paired pigment-cupped ocelli on sides of pharynx.

Nervous System

Cerebral ganglion ("brain") is circumesophageal nerve ring with most of the sensory nerve bodies. Many sensory fibers project anteriorly. Posteriorly, main trunk is in ventral epidermal cord, both motor and sensory nerves. Dorsal nerve trunk is motor, lateral trunks are sensory. All trunks bear ganglia. Many spp. show a "ladder system" of commissures connecting trunks.

Respiration and Gas Exchange

By diffusion (thus restricting size, though pseudocoelom helps overcome this). Some parasitic nematodes have a form of hemoglobin in the body fluids. Both anaerobic and aerobic metabolism is common.

Circulation

No special system. Fluids circulated in pseudocoelom by body movements

Osmoregulation

Uses "renette cells" unique to nematodes, present to varying extents, most developed in free-living forms, most reduced in parasitic ones. Paired renette cells connect to a midventral excretory pore, occasionally in conjunction with a third cell ("ampulla"). Modifications include extensive intracellular ducts within the cytoplasm of the renette cells, extending out into body in H or Y pattern. Some have only ducts. Some spp lack renette cells and have many single-cell units distributed along body, each opening to a pore (possibly noncilliated protonephridia). Cuticle in some forms is differentially permeable to water: allows inflow but not outflow. Most nitrogenous waste is ammonia, lost from walls of midgut. Urea is seen in nematodes in hypertonic environments.

Reproduction

Most are gonochoristic (dioecious) with varying degrees of sexual dimorphism (males usually smaller and with pronounced curvature posteriorly. Fertilization is internal. Double-layered shell is formed around zygotes, which are then released. In some hermaphroditic forms the same organ produces both eggs and sperm (ovotestis), but sperm production precedes egg production ("protandry"). Development of zygote is direct, though juvenile (nonreproductive) forms are called "larvae".

Body Plan	Triploblastic but pseudocoelomate. Body long and filamentous. Unsegmented. Cuticle well developed and complexly layered; secreted by epidermis. Parasitic nematodes have dense fibrous layer under cuticle, not found in most free-living forms. Epidermis cellular or syncytial, often protrudes into coelom as dorsal and ventral cords (housing nerves) and lateral cords (housing nerves and, if present, excretory system). Synapomorphies include muscle cells with filaments extending to dorsal and ventral nerve cords, and special sense organs called phasmids and amphids
Locomotion	Thick layer of obliquely arranged muscle fibers just under epidermis, connected to nerve cords by "muscle arems" (unique to nematodes). Fluid in coelom acts as hydrostatic skeleton. Absence of circular muscles precludes peristaltic movement, so motion is always undulatory; nematodes are poor swimmers and usually require contact with a substrate to move.
Feeding and Digestion	Great variety. Free leiving forms may be carnivorous,detritivouous (some feed on fungi in detritus), scavengers, herbivorous, and bacterivorous. Parasites consume host tissues or body fluids. Digestive tract is complete, with specialized regions: anterior mouth, muscular and glandular pharynx (esophagus), long, straight midgut (intestine), rectum, and ventral anus. Phanyngeal glands (and possibly midgut cells) secrete digestive enzymes, midgut cells have microvilli for absorption.
Sense Organs	Wide variety of setae and papillae which are tactile (mechanoreceptors). Amphids and phaasmids are chemoreceptors unique to nematodes. Some have proprioceptors in lateral epidermal cords to monitor bending. Some free-living forms have paired pigment-cupped ocelli on sides of pharynx.
Nervous System	Cerebral ganglion ("brain") is circumesophageal nerve ring with most of the sensory nerve bodies. Many sensory fibers project anteriorly. Posteriorly, main trunk is in ventral epidermal cord, both motor and sensory nerves. Dorsal nerve trunk is motor, lateral trunks are sensory. All trunks bear ganglia. Many spp. show a "ladder system" of commissures connecting trunks.
Respiration and Gas Exchange	By diffusion (thus restricting size, though pseudocoelom helps overcome this). Some parasitic nematodes have a form of hemoglobin in the body fluids. Both anaerobic and aerobic metabolism is common.
Circulation	No special system. Fluids circulated in pseudocoelom by body movements
Osmoregulation	Uses "renette cells" unique to nematodes, present to varying extents, most developed in free-living forms, most reduced in parasitic ones. Paired renette cells connect to a midventral excretory pore, occasionally in conjunction with a third cell ("ampulla"). Modifications include extensive intracellular ducts within the cytoplasm of the renette cells, extending out into body in H or Y pattern. Some have only ducts. Some spp lack renette cells and have many single-cell units distributed along body, each opening to a pore (possibly noncilliated protonephridia). Cuticle in some forms is differentially permeable to water: allows inflow but not outflow. Most nitrogenous waste is ammonia, lost from walls of midgut. Urea is seen in nematodes in hypertonic environments.
Reproduction	Most are gonochoristic (dioecious) with varying degrees of sexual dimorphism (males usually smaller and with pronounced curvature posteriorly. Fertilization is internal. Double-layered shell is formed around zygotes, which are then released. In some hermaphroditic forms the same organ produces both eggs and sperm (ovotestis), but sperm production precedes egg production ("protandry"). Development of zygote is direct, though juvenile (nonreproductive) forms are called "larvae".