• Phylum Nematoda
• Slender,
unsegmented worm
• Pseudocoelom
• Digestive system with
two openings: a
mouth and an anus
Free-living
roundworms
• Carnivores that use
grasping
mouthparts and
spines
• Exchange gases and excrete
metabolic waste through their body
walls
• No internal transport system
• Depend on diffusion
• Simple nervous systems, consisting of
several ganglia
• Run from the head to the tail
• Nerves transmit sensory information
and control movement
• Hydrostatic skeleton
• Aquatic roundworms move like
snakes
• Soil-dwelling roundworms push
their way through by thrashing
around
• Sexually
• Separate males and females
• Internal fertilization
• Male deposits sperm inside the female’s
reproductive tract
• Parasitic roundworms have complex life cycles
involving two or three different hosts or organs
within a single host
Trichinosis-causing worms
• Adult worms live and mate in the intestines of their
host (humans, pigs and other mammals)
Filarial worms
• Found primarily in tropical regions of Asia,
threadlike worms that live in the blood and lymph
vessels of birds and mammals, including humans,
transmitted by biting insects, causes elephantiasis
Ascarid worms
• Serious parasite of humans and many other
vertebrates, causes malnutrition; spread by eating
vegetables or food that are not washed properly
Hookworms
• Hatch outside the body of the host and develop in
the soil, can enter a barefoot and travel through the
bloodstream to the intestines
Eggs are ingested
Eggs
contaminate
fingers
Larvae hatch in
small intestine
Eggs embryonate Larvae migrate to
on perianum
colon & mature
Larvae enter
lungs & cause
coughing
Larvae penetrate
skin & enter
bloodstream
Gravid adults
migrate out of
anus & lay eggs
Pinworms are the most common worm infection in the
U.S. (over 40 million cases per year—usually in children).
Larvae
migrate
to grass
Larvae hatch &
develop in soil
Larvae are
coughed up
& swallowed
Larvae reach
small intestine,
mature, & start
feeding
Eggs are
passed in
feces and
enter soil
• Free-living roundworm
Caenorhabditis elegans, C.
elegans
• Feeds on rotting vegetation
• First multicellular animal
whose DNA was fully
sequenced
• Helps understand genes
and how eukaryotes
became multicellular
Roundworms
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Pseudocoelomates
Digestive tract with mouth
& anus (tube-within-a-tube)
Cephalized
Mouth
Found in most habitats
Range in size from
Gut
microscopic to several
meters in length
Examples:
 Ascarid worms
Anus
 Hookworms
 Pinworms
 Filarial worms
Mouth
Nerve
ring
Pseudocoelom
Intestine
Cuticle
Excretory pore
Ovary
Ectoderm
Mesoderm
Endoderm
PseudoNematodes coelom
3 germ layers
Pinworm
Heartworm
Anus
Reproductive
pore