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The Pseudocoelomate
Body Plan
Chapter 11
Aschelminths
– Any of the seven phyla
grouped together
– Rotifera, Kinorhyncha,
Nematoda, Nematomorpha,
Acanthocephala, Loricitera,
and Priapulida
Aschelminths
Pseudocoelomate Body Plan
•Lack mesentary, organs lie free
•Often fluid-filled or contain a gelantinous
substance
•Most have complete tubular digestive tract from
mouth to anus which allows for mechanical
breakdown of food, digestion, absorption and
feces formation
Aschelminths
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Most are microscopic (some grow to over a meter)
Bilaterally symmetrical
Unsegmented
Triploblastic
Cylindrical in cross section
Most are dioecious (reproductive organs are in
separate animals)
Aschelminths

Most are microscopic (some grow to over a meter)
 Most are free-living, some are parasitic
 Bilaterally symmetrical
 Unsegmented
 Triploblastic
 Cylindrical in cross section
 Most are dioecious (reproductive organs are in separate
animals)
 Cuticle present: may bear spines or scales and is useful
for protection and taxonomic identification
 Molting or ecdysis shed their cuticle
Aschelminths
Eutely – Same number
of cells for each animal
and for each given
organ
• Ex. Caenorhabditis
elegans (a type of
nematode) has 959
cells
• Every worm has 80
cells in their pharnyx
Page 176
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Nematode Reproductive Systems
Dioecious
Phylum Rotifera
Characteristics
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Corona – ciliated organ around
the head used for locomotion
and food gathering
0.1 to 3 mm in length
Most are freshwater (less than
10% marine)
Phylum Rotifera
Characteristics Cont…
•Usually solitary, free
swimming animals although
there are a few colonial
members
•Posterior end with toes and
adhesive glands
•Parthogenesis common,
males reduced in this phylum
Phylum Rotifera
External features
• Epidermally secreted
cuticle used for
protection
– Lorica – thickened
cuticle that makes an
encasement used for
protection and support
– Epidermis is synctial
– Head has a mouth,
brain, sensory organs
– Foot has 1-2 toes
» Foot has pedal
glands
Phylum Rotifera
Body parts
– Head (Anterior)
» Corona
» Mouth
» Buccal field
– Trunk
» Middle
– Foot
» Toes
» Adhesive glands
Phylum Rotifera
Digestion
• Mastax jaw that grinds
food
Other organs
Protonephridia with flame
cells
• Functions for osmoregulation
Phylum Rotifera
Reproduction
• Some perform sexual
reproduction (several use
parthogenesis)
• Class Seisonida
–
–
–
–
2 Species
Marine
Haploid eggs that must be fertilized
Males & females develop equally
• Class Bdelloidea
– All females are parthenogenic
– Diploid eggs that produce females
– No males present
• Class Monogononta
– Amictic eggs – diploid eggs
– Mictic eggs – haploid eggs, can
become amitic
– Sporadic small sized males
Phylum Kinorhyncha
Characteristics
• < 1mm in length
• Marine environmentsburrow in mud & sand
with snouts
• 150 species
• Dioecious
• Feed on diatom & algae
and organic matter
Phylum Kinorhyncha
• Composed of 13 or 14
zonites
– Definite units called
zonites
– Zonite 1 can retract into
zonite 2
– Spines line most zonites
» Pair of lateral spines and
one dorsal spine
– Protonephridia in Zonite
11
– Brain and ventral nerve
cord with a ganglion in
each zonite
Loricifera
Discovered in 1974
 Dioecious
 Have a large brain
 Little else is known
about them.
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Priapulida
Only 9 species
 All marine worms
 Found in colder
water
 Predaceous
 Fossils date back
to Middle
Cambrian.
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Enterobius vermicularis
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Pin worm
50% of children in US
Spread
– Fecal oral route
– Airborne
 Spends its entire life in humans.
Adult worms are in the large
intestine in humans. The
female migrates to the anus to
deposit her eggs. This causes
the itching that is the most
common symptom. Eggs are
then ingested by the host or
another human (commonly
transmitted in young children
who are not very hygienic).
 Sticky tape method
Phylum Acanthocephala
Spiny headed worm
 2-host parasites

– Must have invertebrate host
Spiny protruding proboscis
 Both circular and longitudinal
muscles
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Nutrition

Nutrition by
diffusion
 Proboscis
attaches to host
intestine
 Cause extensive
damage to the
intestinal walls
 Some forms cause
serious discomfort
and ill-health to
domestic livestock
Characteristics
•
Spiny or thorny-headed
• All are intestinal parasites of vertebrates
• Common in various fishes (mostly freshwater)
• birds (chickens and turkeys)
• mammals and a few reptiles and amphibians.
• Typically cylindrical and small (few mm - cm)
• Constant number of cells, which is species specific
The Acanthocephalan Body
Close up of the proboscis
How the
proboscis
attaches to the
intestinal wall.
All images from:
http://www.biosci.ohiostate.edu/~parasite/acanthocephala.htm
Phylum Nematomorpha

Commonly called horsehair worms
or Gordian worms
 Up to 1m long, but very slender
animals (1-3mm)
 Free-living as adults
 Often find adults in very clean
streams
 Juveniles are parasitic in
arthropods (beetles, cockroach)
Nematodes
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Found everywhere
–
–
–
–
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Soil
Oceans
Polar ice
Hot springs
Parasites of nearly
all plant and animal
species!
Phylum Nematode
Microscopic to several meters long
 Feed on organic matter

– Rotting substances to living tissues of
other invertebrates, vertebrates and plant.
Phylum Nematode
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12,000 species
– 500,000 possible
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Most abundant animal
(some 5 billion may be
in an acre of fertile
garden soil)
Cylindrical body
Only longitudinal
muscles
Noncellular cuticle with
several layers
Phylum Nematode
• Some have lips, some have
spines or teeth on those lips
• Sensory organs
– Amphids – chemoreceptors
along the cuticle
– Phasmids - chemoreceptors
near the anus
– Ocelli – eyespots found in
aquatic nematodes
Ascaris lumbricoides
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Roundworm of man
1.2 billion people
– Many in southeast US
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Females lay 200,000
eggs a day
Unsanitary habits
contaminate ground
Moves by thrashing
back and forth
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Life Cycle of Ascaris Lumbricoides
Source: Redrawn From Centers for Disease Control, Atlanta, GA.
Nematode-Caused Diseases
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Roundworms - more than ½ the world's humans
Hookworms
Trichinosis (Porkworm)
Pinworm infestations - extremely common
parasite in the United States
Filariasis (elephantiasis)
Onchocerciasis (river blindness)
Ascarid Worms (common roundworm)
- lives in intestine
- eggs are passed out in the feces
Most roundworms infect
dogs, but occasionally
they find their way into
human hosts
Enterobius vermicularis

Pinworms
 50% of children in US
 Spread
– Fecal oral route
– airborne
Enterobius vermicularis
Spends its entire life in humans. Adult worms
are in the large intestine in humans. The female
migrates to the anus to deposit her eggs. This
causes the itching that is the most common
symptom. Eggs are then ingested by the host or
another human
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Life Cycle of Enterobius Vermicularis
Source: Redrawn From Centers for Disease Control, Atlanta, GA.
Hookworms
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Anterior end hooks
Feed on blood
Cause anemia
Necator americanus
Ancylostoma duodenale
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Life Cycle of Necator Americanus
Source: Redrawn From Centers for Disease Control, Atlanta, GA.
Ancylostoma caninum
www.animalplanet.com/s/3336/157?showName=Monsters%20InsideMe&video
Episode=Worms%20Crawling%20Under%20My%20Skin
Cutaneous Larval Migrans
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Hookworms from
dogs and cats
Trichinella spiralis
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Trichina worm: causes
trichinosis
cysts within the muscles are
consumed (undercooked food)
worm grows in intestine

forms cysts in the muscles of
the new host
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symptom: terrible pain in
muscles
Diagnosis is by muscle biopsy
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Life Cycle of Trichinella Spiralis
•Acquired by ingestion of
contaminated pork
containing the encysted
larva.
•The larvae mature into
adults in the human
digestive tract.
•They sexually reproduce
and give birth to live
nematodes that migrate
throughout the body and
become encysted in
muscles and other tissue.
•Infects pigs, bears, dogs,
cats, rats & humans
Source: (a) Redrawn From Centers for Disease Control, Atlanta, GA. (b) Photo © Steve Miller
Dirofilaria immitis
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Dog heart worm
Wuchereria bancrofti
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Filarial worms- mostly in
tropical regions
Infect the lymph vessels
which are responsible for
returning fluid to the
circulatory system
Obstruct lymph to cause
swelling
– Elephantiasis
-- usually transmitted
by mosquitoes
-- causes elephantiasis
Elephantiasis
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Life Cycle of Wucheria spp.
11-13
Source: Redrawn From Centers for Disease Control, Atlanta, GA.
Loa loa
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Eye worm
 Can cause
encephalitis
Dracunculus medinesis

Fiery serpent
Necator americanus – hookworm. Adults live
in the small intestine and eggs are excreted
with feces. They hatch in the soil and the
larva can enter a new host by penetrating the
skin. The go to the blood and the lungs where
they are swallowed and get to the small
intestine to mature into an adult. Diagnosis is
by finding eggs in the feces. To prevent
infection – wear shoes