Unit 5 Notes #5 Phylum Aschelminthes (Nematoda)
ROUNDWORMS
A) Body Plan:
-Unsegmented worms that are cylindrically shaped.
- Most are free-living, but our focus will be on those that
are parasitic.
Ascaris Shown above
Similarities to Platyhelminthes:
1. Bilateral symmetry
2. Three tissue layers (mesoderm specialized to give rise
to muscle and various internal organs)
3. Tegument and cuticle (protection)
Differences :
- Complete digestive tract (mouth to anus).
-Between the mesoderm and the gut is a fluid filled
space called a Pseudocoelom (“False Cavity”)
B) Continuity-Reproduction:
-Sexes are separate in most roundworms
- In the female, there is a pair of long, thin, coiled
ovaries that produce thousands of eggs every day.
- In the male, there is one testicle; its job is to produce
sperm.
- After the eggs are fertilized, shell glands secrete a shell
around the egg so when they are released they will be
protected.
C) Various Systems:
1. Digestive:
-Aschelminthes have a complete digestive system so
materials move in one direction only. – More efficient
-As food is pushed through the digestive tract, it is
digested and nutrients are absorbed. Undigested
remains continue on through the tract and are
eventually eliminated out the anus.
2. Circulation & Respiration
- No circulatory or respiratory systems.
- Breathe and excrete metabolic wastes through their
moist skin.
- The fluid in the pseudocoelom distributes digested
foods and dissolved oxygen.
D) Advances of Aschelminthes over Platyhelminthes:
1) They have a complete gut; more efficient digestion.
2) They are Dioecious (either male or female) and they
only use sexual reproduction.
3) They have a fluid filled body cavity (pseudocoelom),
which contains their internal organs. The fluid in this
cavity helps distribute materials and also provides a
hydraulic (water) skeleton to push muscles against.
E) Roundworm Human Disorders
1) Parasitic blisters: “Guinea worms” form ulcerating
blisters in human skin.
2) Elephantiasis: The “Filarial worm” blocks lymph
vessels. This causes massive swelling and deformities.
Eye worms are a close relative of Filarial worms. Most
often transmitted by biting insects (mosquitoes).
Elephantiasis
Eye worms
3) Parasitic Anemia: “Hookworm”: Larvae invade
Human hosts by using hooks to burrow through the
skin of bare feet to find their way into blood vessels.
From there, they circulate up to the lungs where they
burrow into the air passages of the lungs. They then
travel to the throat, where they are swallowed and come
to anchor into the intestines.
- These parasites feast on
blood, leaving the host
(human) short of blood
therefore short of oxygen
due to a reduction in the
number of red blood cells.
Hookworm Mouth
- Many such hookworms belong to the Genus “Ascaris”
Bolus of Ascaris worms rectally passed
4) Trichinosis: -From the “Trichinella worm”
- Invade host when host eats uncooked meat (pork)
- Set up in the intestine.
- After mating the worms can produce up to 1500
microscope larvae which burrow into the host’s blood
vessels.
- After traveling in the blood vessels they burrow into
the host’s (pig, human, rats etc) tissues (muscles,
organs). There they form painful cysts.