Download phylum nematoda

PHYLUM NEMATODA
y Nematodes, also known as round worms, are among
the simplest animals to have a complete digestive
system; ie: they have both a mouth and an anus.
y Nematoda is the second largest of the animal phyla,
and its members are more numerous than those of
any other phylum of multicellular animals.
Habitat
y Most are free – living and are found in virtually all
parts of the earth. Many are parasites that affect
almost every kind of plant and animal. The variety
of habitats occupied is greater than all other animal
groups except arthropods.
Structure
y Elongate, cylindrical body that contains a fluid-filled
body cavity called the pseudocoelom between the
body wall and the digestive tract.
y The pseudocoelom acts as both a hydrostatic
skeleton upon which the muscles can act, and a
transportation system for nutrients, oxygen and
wastes.
y Tapered body, more blunt at the front end, is covered
with a tough cuticle.
Ingesting Food
y Free-living forms are often carnivorous and catch
and eat small animals with their muscular sucking
pharynx. Some feed on algae, fungi, and pieces of
decaying organic matter in the soil. Others are
parasitic and feeding on living plant and animal
tissues.
y May have a piercing stylet for sucking out and
digesting cell contents, or sharp teeth and cutting
plates for digesting intestinal linings.
Digesting Food
y Food is broken down both chemically and physically
in the straight digestive tract that opens through the
mouth and exits through the anus. Food moves
through the intestine by ingestion of more food and
the general movements of the worm’s body, rather
than by muscular contractions.
Elimination
y Undigested food exits via the anus. Muscles
surrounding the anus relax, and the waste is
forcefully expelled. Very efficient unidirectional
movement of food through the digestive system is a
significant evolutionary step.
Internal Transport
y Nutrients, oxygen and wastes are transported
through the fluid in the pseudocoelom. No internal
transport system exists, so the roundworms depend
on diffusion to move materials through their body.
Respiration
y Breathe through their body walls; rely on diffusion,
no internal transport system.
Excretion
y The digestive tract seems to play a major role in
excretion, and the excretory system probably
function in osmotic regulation. Excretory wastes will
also move by diffusion through the pseudocoelom.
Locomotion
y Depends on the opposing forces of the longitudinal
muscles, elastic cuticle, and hydrostatic skeleton.
Range of motion is limited by the lack of circular
muscles and the way the nervous system controls
muscle contraction: the body only bends in the
dorsoventral plane resulting in a whip like thrashing
motion.
Sensing Environment
y A variety of sense organs in the form of tactile
bristles and chemosensory pits can be found. Simple
photoreceptors have been found in some marine and
freshwater nematodes.
Coordination
y Nervous system consists of a nerve ring and ganglia,
around the pharynx, from which nerves run both
forward and backward. Nerve ring is the site of most
sensory integration. Usually 4 major nerve cords.
Reproduction
y Reproduce sexually. Most species have separate
males and females, but a few are hermaphroditic.
Fertilization takes place inside the body of the
female.
y As is usual, parasitic species have complex life cycles
that can involve several hosts.