Pseudocoelomate Animals
There are 9 pseudocoelomate phyla.
These are a diverse lot most of which are small,
and some microscopic, although a few are
relatively large.
Most are free living although some are
exclusively (Acanthocephala) parasitic or include
many parasitic species (Nematoda).
Pseudocoelomate Animals
All have a pseudocoelom, a cavity
surrounding the gut.
Unlike a true coelom, a pseudocoelom is
not a cavity surrounded by mesoderm.
Instead it is a persistent blastocoel and
lacks a peritoneum.
Pseudocoelomate Animals
All pseudocoelomates have a body wall of
muscles and epidermis that surrounds the
pseudocoel.
All pseudocoelomates except for the
Acanthocephala have a complete gut.
Pseudocoelomate Phyla
The pseudoceolomate phyla are listed below. In
the interests of time we will discuss only the first
three:
Rotifera: wheel animals
Acanthocephala: spiny-headed worms
Nematoda: roundworms
Gastrotricha
Entoprocta
Nematomropha
Kinorhyncha
Priapulida
Loricifera
Phylum Rotifera
Rotifers are named for their characteristic
ciliated crown or corona, which when it beats
looks like a rotating wheel.
Rotifers are tiny animals (most are 100-300µm
long and the largest only 3mm long) the majority
of which live in freshwater and are benthic
inhabitants (live on the bottom).
About 2000 species have been described.
Figure 15.18
9.1
Phylum Rotifera
The beating of the cilia in the corona draws in
plankton-containing water for food.
The mouth opens to a modified muscular
pharynx known as a mastax, which is a
structure unique to rotifers.
The mastax has a set of complex jaws, which
are used to grasp and chew food.
Phylum Rotifera
One group of rotifers, the Bdelloid rotifers,
are very unusual in that there are no
males, hermaphrodites, or evidence of
meiosis.
Molecular evidence suggests that there
has been only asexual reproduction in this
group for several million years.
Phylum Rotifera
Because of the problem of accumulation of
deleterious mutations in lineages of exclusively
asexually reproducing animals (a process known
as Muller’s ratchet) it is unclear how the
bdelloids have been able to dispense with
sexual reproduction entirely.
Other rotifers reproduce using a combination fo
sexual and asexual reproduction.
Phylum Acanthocephala
Acanthocephalans are commonly known
as spiny-headed worms because of the
spiny eversible proboscis they use to
attach to the gut of their host.
All 1100 species of Acanthocephalan are
endoparasitic and most parasitize fish,
birds and mammals.
Figure 15.20
9.3
Phylum Acanthocephala
The body wall is covered with numerous
minute depressions which enormously
increase the surface are of the tegument
and facilitates (as in cestodes) the
absorption of food from the host’s gut.
As is true in cestodes, Acanthocephalans
lack a gut.
Phylum Acanthocephala
Acanthocpehalans have a lifecycle in
which a vertebrate is the definitive host
and an invertebrate the intermediate host.
Acanthocephalans, as other parasites do,
modify the behavior of the intermediate
host to enhance the chances of its being
eaten.
Phylum Acanthocephala
For example, acanthocephalans that parasitize
Gammarus, a small freshwater crustacean,
cause the Gammarus to alter its behavior in the
presence of ducks, a common predator.
Instead of diving to the bottom when a duck
appears, the Gammarus swims into the light and
grasps tightly onto a piece of vegetation, greatly
increasing its chances of being eaten.
Phylum Acanthocephala
The change in behavior appears to be caused
by the Acanthocephalan pumping a serotonin-
boosting molecule into the Gammarus’ brain.
This causes the Gammarus to think it’s having
sex and cling as it would if mating. Interestingly,
the parasite’s manipulation also causes female
Gammarus to mimic the males mating behavior.
Phylum Acanthocephala
Another Acanthocephalan that parasitizes pill
bugs causes them to reverse their normal
behavior and avoid humid, dark areas.
Instead they wander in the open where they are
much more vulnerable to birds, the
acanthocephalans definitive host.
The parasite’s manipulations are very effective.
Although fewer than 1% of pill bugs are typically
infected with acanthocephalan parasites 30% of
pill bugs delivered to nestlings are infected.
Phylum Nematoda
The nematodes are by far the most
important group of pseudoceolomates
both in terms of numbers (about 10,000
species) and their impact on humans.
Most nematodes are under 5cm and many
are microscopic. However, some parasitic
forms can be over a meter in length.
Phylum Nematoda
Nematodes use their pseudocoelom as a
hydrostatic skeleton.
The body has a thick cuticle (made
primarily of collagen) secreted by the
underlying epidermis, which resists the
high hydrostatic pressure exerted by the
fluid in the pseudocoelom.
Phylum Nematoda
Beneath the epidermis is a layer of longitudinal muscles.
Muscles in nematodes are not arranged in antagonistic
pairs, the antagonistic role is played by the cuticle.
Contraction of a longitudinal muscle on one side is
transmitted through the hydrostatic skeleton and
stretches the cuticle on the opposite side of the body.
When the muscle relaxes, the cuticle contracts and the
body returns to its resting position.
Phylum Nematoda
Nematodes have a complete gut with a mouth,
muscular pharynx, intestine, rectum, and anus.
Most nematodes are dioecious and males are
smaller than females.
Fertilization is internal and juveniles go through
several developmental stages, each time
molting or shedding their cuticle.
Free-living nematodes
Free-living nematodes live in the sea, in fresh
water, and in the soil. They occur worldwide in
all environments and most live in the interstitial
spaces of sediments and soils.
Vast numbers of nematodes occur. One square
meter of sea bottom mud has been estimated to
hold 4.4 million nematodes and 90,000 were
counted on a single decomposing apple.
Free-living nematodes
The slender, tapered body of nematodes
equips them to live in interstitial spaces.
Most free-living nematodes are less than
2.5mm in length and often are
microscopic. The largest soil dwelling
nematodes may be 7mm long and the
largest marine forms a whopping 5cm.
Free-living nematodes
Most free-living nematodes are
carnivorous.
However, some feed on algae and fungi
and some are detritivores. Others feed on
plants, especially the roots.
Free-living nematodes
Many root feeding nematodes are major
agricultural pests. These species pierce
root cells and suck out their contents.
Nematodes are estimated to destroy 12%
of the world’s cash crops annually.
Parasitic nematodes
There are a great many species of
parasitic nematodes and they attack
virtually all groups of animals and plants.
Parasitic forms include ascarids,
hookworms, Guinea worms, trichina
worms, pinworms, and filarial worms.
Ascaris lumbricoides: large
roundworm of humans
It’s estimated that worldwide as many as 1.4
billion people are infected with Ascaris
lumbricoides which lives in the small intestine.
Females may be a foot long and produce
200,000 eggs a day.
Infection occurs when parasite eggs are eaten
with uncooked food or when soiled fingers are
put into the mouth.
Ascaris lumbricoides: large
roundworm of humans
The larvae penetrate the intestinal wall
and travel through the blood stream to the
lungs where they break out of the alveoli
(often causing pneumonia).
Then they make their way up the trachea
where they are swallowed and eventually
settle in the small intestine.
Ascaris lumbricoides: large
roundworm of humans
In the intestines the worms cause
abdominal symptoms and allergic
reactions and may produce an intestinal
blockage.
Figure 15.05a
Male (top) and 9.8 Ascaris lumbricoides
female
Hookworms
Hookworms are named for the dorsal
curve in their anterior end.
Hookworms are quite small, the
commonest species Necator americanus
is only 11mm long. However, because
they feed on blood a heavy infection can
produce severe anemia.
Hookworms
Large plates in the hookworm’s mouth are used
to cut the intestinal lining of the host.
The parasite then pumps blood through its gut,
partially digesting it before excreting it.
Because hookworms suck more blood than they
use, they can cause debilitating anemia. In
children a hookworm infection can stunt growth
and cause a general lack of energy.
Figure 15.06
9.9
Section through hookworm
attached to dog intestine
Hookworms
Hookworms do not permanently attach in
one spot, but move around the gut and
reattach when they are ready to feed.
Hookworms have evolved sophisticated
anti-clotting factors that keep platelets
from clumping and forming a clot while the
hookworm is feeding.
Hookworms
When the hookworm releases, a clot forms
and the tissue can recover.
By using this approach instead of
producing a crude blood thinner to ensure
blood flow, hookworms prevent hemophilia
developing in their hosts, which would be
fatal for the hookworm.
Hookworms
The life cycle of hookworms is very similar
to that of ascarids and infection occurs in
the same way by contact with eggs in soil
or food.
Guinea worms
Guinea worm infections (also referred to as
Dracunculiasis) are now confined to sub-
Saharan Africa. Adults are threadlike nematode
worms that can grow to 1 meter in length.
The adult lives in humans and the intermediate
host is tiny crustaceans.
Humans become infected when they drink water
containing the crustaceans.
Guinea worms
The immature worm penetrates the gut
wall and wanders through the body,
maturing and growing.
After about a year the female makes her
way to the surface of the skin (usually in
the legs) causing very painful blistering.
Guinea worms
To ease the pain, sufferers immerse their
feet in water. This bursts the blisters and
the female worm then protrudes from the
sore and lays her eggs, thus continuing
the life cycle.
Guinea worms
There is no cure for Guinea worms and
the only way to remove one is to slowly
over the course of weeks wind the worm
out on a stick.
If the worm breaks,
a serious bacterial
infection results.
Interestingly, the traditional symbols for medicine
and healing the staff of Asclepias (showing a
snake entwined around a staff) and the caduceus
(which shows two snakes entwined about
a winged staff) very likely are derived from the
Guinea worm removal technique.
Guinea worms
Guinea worm infection is avoidable with
relatively simple precautions such as
preventing people walking in drinking
water sources and boiling or filtering water
before drinking it.
Guinea worms
Since the mid 1980’s a campaign to
eradicate Guinea worms coordinated by
the U.N. and the Carter Center has had
tremendous success.
In 1986, an estimated 3.5 million people
were infected, but by 2000 the number of
cases had been reduced to about 75,000.
Guinea worms
Guinea worms have been eliminated from
Pakistan, India, and Iran and infections
greatly reduced over much of sub-Saharan
Africa.
The major barrier to elimination at this
point is the ongoing conflict in southern
Sudan where the majority of cases now
occur.
Trichina worms
Trichinella spiralis is a tiny nematode that
causes the potentially fatal disease
trichinosis.
Humans typically become infected by
eating undercooked pork. Trichinella lives
in cysts formed in individual muscle cells
of the host.
Trichina worms
Trichinella when it hatches from an ingested
cysts in its host’s gut drills through the wall of the
gut where females produce living young.
These juveniles travels in the circulatory system
to a muscle.
The juvenile penetrates an individual muscle cell
and breaks the cell down so it can be remade.
Trichina worms
Trichinella, just as a virus does, manipulates the
host cell’s DNA. It causes the cell to recruit a
blood supply to supply food to the cell and also
produce collagen to form a cyst around the cell.
The Trichinella juvenile awaits ingestion by
another host. When ingested it emerges from its
cysts enters the mucosal lining of gut, develops
into an adult and continues the life cycle.
Trichina worms
Adults usually do not persist long in the
gut before being expelled by the host’s
immune system.
Trichina worms
Humans are an inadvertent host of
Trichinella.
Normal life cycles involve pigs and rats.
In humans, infection with a few Trichinella
parasites may cause no symptoms, but
heavy infections can cause intense muscle
pain and in some cases death.
Filarial worms
At least 8 species of filarial worms infect
humans especially in tropical regions.
Approximately 250 million people infected
with Wuchereia bancrofti or Brugia malayi
These worms live in the lymphatic system
and females can be 10cm long.
Filarial worms
Females release live young microfilariae
into the blood and lymphatic system.
The microfilariae are picked up by
mosquitoes where they develop, become
infective and can infect another person.
Filarial worms
In some people exposed to persistent
infections with these parasites,
elephantiasis may develop.
This is caused by blockage of lymphatic
ducts and inflammation. There may be
excessive growth of connective tissue and
enormous swelling of infected parts
including legs, arms and scrotum.
Figure 15.11
Elephantiasis of leg
caused by filarial worms
9.12
Filarial worms
The most common filarial worm in the U.S.
is the cause of heartworm.
The microfilariae are transmitted by
mosquitoes.
Figure 15.12
Diriofilaria immitis
Dog heartworm
9.13
River blindness
River blindness is also caused by filarial worms.
In this case the insect that transmits the disease
is a blackfly.
18- 30 million people are infected worldwide
(mainly central Africa and parts of South
America) and more than 300,000 have been
made blind.
River blindness
When a black fly becomes infected, the worm
larvae spread to its salivary glands. When it
bites someone the larvae pass into the skin.
The larvae develop into adults and form nodules
under the skin. The adults breed and produce
thousands of larvae, which spread all over the
body - including the eyes.
River blindness
The worst problem is caused when
problem is when the parasites die. The
immune system produces a severe
inflammation, which if it occurs in the eye it
can cause blindness.
People infected at birth with river
blindness commonly become blind by their
40s.