Bilateria


Having completed the radially symmetrical
Cnidarians and Ctenophores we now move
on to the remaining animals, all of which
are bilaterally symmetrical (or secondarily
radially symmetrical [the Echinoderms]).
These are the Bilateria.
Bilateria



All Bilateria are triploblastic and belong to two
major groups, which we have already met, the
Protostomia and the Deuterostomia.
Deuterostomes include the Echindoerms,
Hemichordates and Chordates.
Protostomes include all the other bilateral
invertebrates including Platyhelminthes,
Annelida, Mollusca, and Arthropoda.
Protostomia divisions

Classification of invertebrates is in a state of flux for several
reasons:



molecular phylogenetic studies have cast doubt
on traditional invertebrate classification based on
morphological characteristics.
many invertebrates are soft-bodied so fossils of
many groups are rare or unknown, which makes
relationships between groups hard to decipher.
Traditional major groups of protostomes
(acoelomate, pseudocoelomate and coelomate)
appear not to be monophyletic.
Protostomia divisions

For this class, we will use the
Deuterostome/Protostome arrangement
and recognize two majors groups of
Protostomes: the Lophotrochozoa and
the Ecdysozoa.
Protostome Divisions: the
Lophotrochozoa and Ecdysozoa.


Protostomes are divided into two large
groups the Lophotrochozoa and the
Ecdysozoa.
The relatedness of phyla within these two
groups is not entirely clear and will likely
change in the future.
Lophotrochozoa and Ecdysozoa


Lophotrochozoa: members generally possess a
trochophore larva [free-swimming oval or
pyramidal ciliated larva with a band of cilia
around the body] or a lophophore [tentacle
bearing arm which contains within it an
extension of the coelomic cavity].
Ecdysozoa: members shed their cuticle as they
grow
Trochophore larva
http://www.microscopy-uk.org.uk/mag/imgjan09/image006.jpg


Trochophore larva clip
https://www.youtube.com/watch?v=Z0cE
OvGcrl8
Bryozoan lophophore
http://www.bryozoans.nl/pictures/
figuren/anatomy.jpg
Lophophore: characteristic feeding structure of members of the
Brachiopoda, Bryozoa and Phoronida


Lophophore clip
https://www.youtube.com/watch?v=Op3FEVwa0E
Lophotrochozoa and Ecdysozoa

Lophotrochozoa members:


Platyhelminthes, Annelida, Mollusca, and a
diverse array of “lesser phyla” including
Nemertea, Rotifera, Acanthocephala,
Brachiopoda, and Sipuncula.
Ecdysozoa members:

Nematoda, Arthropoda and “lesser phyla”
including Onychophora, Tardigrada,
Kinorhyncha, and Priapulida.
Eutrochozoa: Rotifera, Acanthocephala,
Entoprocta,Platyhelminthes, Nemertea, Mollusca,
Sipuncula, Annelida
Lophotrochozoa
Lophophorata: Ectoprocta, Brachiopoda,
Protostomia
Phoronida
Ecdysozoa: Onychophora, Tardigrada,
Arthropoda, Nematoda, Nematomorpha,
Kinorhyncha, Loricifera, Priapulida
Cuticulata
Gastrotricha
Parenchymia
Platyhelminthes
Nemertea
Annelida
Mollusca
Sipuncula
Entoprocta
Rotifera
Lophotrochozoa
Acanthocephala
Ectoprocta
Lophophorata
Brachiopoda
Phoronida
“Worms”


The term worm is loosely employed in biology
and is applied to very different animals including
the segmented worms (Annelids), roundworms
(Nematoda) ribbonworms (Nemertea) flatworms
(Platyhelminthes) and others.
“Worm” describes any bilaterally symmetrical,
legless, soft-bodied animal at least 2-3 times as
long as it is wide.
Phylum Platyhelminthes

Unlike other animals encountered so far,
Platyhelminthes:




have evolved cephalization with their sense
organs concentrated at the head end.
Have a ladder-type nervous system.
are bilaterally symmetrical.
Are dorsoventrally flattened.
Phylum Platyhelminthes

In addition they are



triploblastic, but lack a coelom. Instead, they
have a solid body filled with parenchyma cells.
have evolved organs and in some cases organ
systems.
The first (and simplest) excretory or
osmoregulatory systems and circulatory
systems are found in members of these
groups.
Phylum Platyhelminthes



Platyhelminthes usually slender and leaflike or
ribbonlike. Unsegmented.
Have no circulatory or respiratory organs. Flat
body increases surface area and allows the
animal to exchange gas and lose wastes by
diffusion.
Four classes in the Platyhelminthes. Turbellaria
are free living but the Monogenea, Trematoda
and Cestoda are parasitic.
Nutrition



The digestive system includes a mouth, pharynx,
and blind intestine (the gut is incomplete).
In the free-living Turbellarians the pharynx can
be everted from the mouth.
Food is sucked into the intestine where a
combination of extracellular and intracellular
digestion takes place.
http://www.thaigoodview.com/library/contest2551/science04/119
/kingdon_animalia/images/turbellaria4.jpg
“Organization of the blind digestive cavity of polyclads [a group of Turbellarians]
with highly branched diverticles (ventral view)”
http://www.rzuser.uni-heidelberg.de/~bu6/Introduction04.html
Nutrition


Undigested food exits via the pharynx.
In the Cestoda the digestive tract is
absent and all nutrients are absorbed
across the tegument (the syncytial
membrane/body covering found in all
parasitic Platyhelminthes).
Excretion/Osmoregulation



The osmoregulatory system consists of a series of canals
that end in flame cells or protonephridia.
The flame cell consists of a fine-meshed cup that
contains cilia. The beating of the cilia draws fluid which
is filtered as it passes into the cup.
This system appears mainly intended to remove excess
fluid, but retain essential ions. It is most developed in
freshwater Turbellarians, but reduced or absent in
marine species, which do not have to remove excess
water.
http://www.cartage.org.lb/en/themes/Sciences/Lifescience/GeneralBiology/
Physiology/ExcretorySystem/Invertebrate/flatwormexcret.gif
Flame cells at 1000x
magnification

https://www.youtube.com/watch?v=Rb_3
KIB4CmE
Nervous system and sense organs

Flatworms possess a simple brain and one to
five pairs of longitudinal nerve cords that are
cross connected to form a ladder-like
arrangement.
Nervous system of Dugesia
http://biodidac.bio.uottawa.ca/ftp/BIODIDAC/ZOO/PLATYHEL/DIAGCL/TURB007C.GIF
Nervous system and sense organs


Neurons are specialized for different tasks
e.g. sensory and motor functions, which is
an important advance in the evolution of
nervous systems.
There are a number of different sensory
cells found in flatworms and tactile and
chemoreceptive cells are abundant.
Nervous system and sense organs


In freshwater Planarians concentrations of
sensory cells form two ear-like structures
(the auricles) found on the side of the
head.
Light sensitive eyespots or ocelli are
common in all classes but Cestoda.
Freshwater Planarians:
http://www.aecos.com/CPIE/flatworm.jpg
Reproduction


Reproduction in the Platyhelminthes can
be asexual or sexual. However, most are
hermaphroditic and cross fertilize.
In parasitic forms sexual and asexual
reproduction may alternate in different
stages of the life history
Penis fencing in Turbellarians

https://www.youtube.com/watch?v=wn3xl
uIRh1Y
Classification of Platyhelminthes

There are four classes in the
Platyhelminthes:




Class Turbellaria: free-living flatworms.
Class Trematoda: endoparasitic flukes
Class Monogenea: parasitic flukes that are
mainly ectoparasites
Class Cestoda: tapeworms
Class Turbellaria



Class Turbellaria contains about 3000 species.
There is considerable debate about the
classification of the class and it is likely that the
class is not monophyletic.
Most species are marine and benthic (move
around on the bottom in aquatic environments).
Some also found in fresh water or in moist
temperate and tropical terrestrial habitats.
Figure 14.10
8.2
Marine turbellarian
Dugesia tigrina, a freshwater turbellarian
© Mauricio A. Muñoz
Class Turbellaria


Most Turbellarians are predators of invertebrates
smaller than themselves. A few are herbivores
or scavengers.
In many species the pharynx is protrusible and
can be inserted into the prey to begin digesting
it. Some species stab prey with their penis (it
has a hardened tip or stylet).
Predatory flatworm hunting
snails.

https://www.youtube.com/watch?v=3DU_
pvAtIYQ
Class Turbellaria


Turbellarians move by swimming, creeping or
crawling. They combine muscular contractions
with ciliary movement to move.
Turbellarians may also use waves of muscle
movement to move.
Swimming marine turbellarian

https://www.youtube.com/watch?v=7UkZ
HDIujUc
“Polycladida moseleyi is distributed throughout the Mediterranean Sea and the
temperate eastern Atlantic. Its favored food are tunicates (Clavelina sp.). “
http://www.rzuser.uni-heidelberg.de/~bu6/flat0431.html
Class Trematoda


There are about 9000 species of
trematodes (flukes) all of which are
parasitic. Most parasitize vertebrates.
Adaptations for parasitism include suckers
and hooks for attachment, glands to
produce cyst material and increased
reproductive capacity.
Sheep liver fluke
Class Trematoda

Structurally trematodes are similar to
turbellarians having a well developed
digestive system and similar nervous,
excretory, and reproductive systems.
However, a major difference is the
tegument.
Tegument



The tegument (found in all parasitic
Platyhelminthes) is a nonciliated, cytoplasmic
syncytium that overlays layers of muscle.
The syncytium represents extensions of cells
that are located below the muscle in the
parenchyma.
The tegument protects the parasite against its
host (e.g. against digestive enzymes).
Figure 14.05
8.5
Digenean Trematodes


There are three subclasses of Trematodes,
but two are small, poorly studied groups.
The third group, the Digenea, however is
a large group of major medical and
economic importance.
Digenean Trematodes


Flukes have a complex life cycle in which a
snail is the first (or intermediate) host and
a vertebrate the final (or definitive host).
The definitive host is one in which the
fluke reproduces sexually.
Digenean Trematodes


In some species there may be 2 or 3
intermediate hosts before the definitive
host is reached.
Trematodes inhabit a variety of sites in
their hosts including the digestive tract,
respiratory tract, circulatory system,
urinary tract, and reproductive tract.
Digenean Trematodes

Digenean life cycles are very complex and
the fluke passes through numerous
stages.
Digenean Trematodes

A typical example would include the
following stages:



Adult
Egg (or shelled embryo) shed into water
Miracidium: a free swimming, ciliated larva
that finds and penetrates a snail intermediate
host
Hatching schistomsome
miracdia

https://www.youtube.com/watch?v=qPqW
TY80UzE
Digenean Trematodes



Sporocyst: reproduces asexually in
intermediate host producing more sporocysts
or another asexually reproducing stage called
a redia.
Redia produce more redia or cercariae.
Cercariae leave the intermediate host and
swim. Then they penetrate the skin of
another intermediate host or the definitive
host.
Cercariae

https://www.youtube.com/watch?v=Dy6S
hA04qT0
Digenean Trematodes


Cercariae that enter an intermediate host
encyst in muscle and wait to be consumed by
the definitive host.
Cercariae that enter the definitive host make
their way to their desired home and develop
into an adult fluke which reproduces sexually
and produces eggs.
Clonorchis liver fluke



Clonorchis is the most important liver fluke to infect
humans. Common in much of Asia (including China,
Japan and southern Asia). Estimated 30 million people
infected.
Adult flukes live in the bile passages and shelled miricidia
pass out in feces. The miricidia enter snails eventually
leave the snails as cercariae and find a fish where they
encyst.
If fish is eaten raw or poorly cooked the person becomes
infected
Figure 14.12
8.8
How do flukes manipulate their
hosts?


Many parasites infect an intermediate host that
needs to be eaten by the definitive host for the
parasite to complete its lifecycle.
There are many instances of parasites altering
their intermediate hosts behavior to make it
more vulnerable a predator (the definitive host).
Such behavior is widespread in flukes.
How do flukes manipulate their
hosts?


The fluke Euhaplorchis californiensis lives
in the Carpenteria Salt Marshes in
southern California.
Life cycle includes two intermediate hosts,
first the California Horn Snail and then the
California killifish and a final host: fish
eating birds e.g. gulls.
How do flukes manipulate their
hosts?



Fluke eggs contained in bird droppings which
are eaten by the fluke’s first intermediate host
the snail.
The fluke castrates the snail (so host does not
“waste” energy reproducing) leaving more
resources for the fluke to use.
Fluke then reproduces asexually in snail and
sheds cercariae into the water.
How do flukes manipulate their
hosts?


Cercariae seek out the next intermediate
host the killifish and latch onto the fish’s
gills.
Each cercaria works its way into a blood
vessel, then seeks out a nerve and follows
it to the fish’s brain.
How do flukes manipulate their
hosts?


Cercariae don’t penetrate the brain but sit
on top of it and wait for the fish to be
eaten by a bird.
Once eaten by a bird cercariae move into
the bird’s gut, transform into adults and
produce eggs that continue the cycle
How do flukes manipulate their
hosts?


Cercariae on the fish’s brain manipulate
the fish’s behavior.
Killifish when swimming sometimes
shimmy and jerk around flashing their
bellies. Fish infected with cercariae are
four times more likely to do so than noninfected fish.
How do flukes manipulate their
hosts?

In field experiments in which penned fish
were made available to foraging birds
infected fish were 30 times (!) more likely
to be eaten than uninfected fish.
How do flukes manipulate their
hosts?


Research has shown that the flukes
produce powerful molecular signals called
fibroblast growth factors.
These interfere with the growth of nerves
and may be the mechanism the flukes use
to alter the fish’s behavior.
Schistosomiasis


Schsitosomiasis is an infection with blood
flukes and is one of the most important
major infectious diseases on the planet.
More then 200 million people are infected
worldwide with these flukes which they
acquire swimming or walking in water in
which the intermediate snail host lives
"Schistosomiasis Life Cycle" by Unknown - CDC DPDx. Licensed under Public Domain via Commons https://commons.wikimedia.org/wiki/File:Schistosomiasis_Life_Cycle.png#/media/File:Schistosomiasis_L
ife_Cycle.png
Schistosomiasis


When a schistosome cercaria swims it avoids UV
light which can damage it, but is very sensitive
to the scent of humans.
When it senses molecules from human skin it
swims rapidly and jerks around looking for the
person. When it makes contact it releases
chemicals that soften the skin and burrows in
shedding its tail at the same time.
Schistosomiasis


The fluke searches until it finds a capillary and
enters it. It moves along using its pair of
suckers.
Eventually, it makes its way to the liver.
Schistosomiasis



In the liver, the fluke feeds on blood and
grows and becomes sexually mature.
It attracts a mate by chemical signaling.
Females are slender and delicate, whereas
males are much bigger and have a spiny
trough or groove into which the female
fits and locks in.
Figure 14.13
8.9a and b
Schistosomiasis


Once paired up the flukes travel from the
liver to a permanent home that is speciesspecific.
In Schistosoma mansoni it is near the
large intestine, in S. haemotobium it is the
bladder, and in S. nasale, a blood fluke of
cows, it is the nose.
Schistosomiasis


Once established the pair remain in situ
for the rest of their lives.
The male consumes blood and feeds the
female most of it, which she turns into
eggs, which pass out of the host and can
begin the life cycle again.
Schistosomiasis



Causes an estimated 12,000-200,000
deaths annually.
Causes anemia and malnutrition. Stunts
growth and brain development in children.
Liver damage, kidney failure, infertility and
bladder cancer can result from infection.
Schistosomiasis

Enlargement of the abdomen due to
swelling of the liver is common.
Carter Center schistosomiasis
video

https://www.youtube.com/watch?v=5O0k
r7oW-6k
Do trematode parasites favor sex in
hosts?



Lively (1992) studied New Zealand
freshwater snail. Host to parasitic
trematodes.
Trematodes eat host’s gonads and
castrate it which imposes strong selection
pressure.
Snail populations contain both obligate
sexually and asexually reproducing
females.
Do trematode parasites favor sex in
hosts?


Proportion of sexual vs asexual females
varies from population to population.
Frequency of trematode infections varies
also.
Do trematode parasites favor sex in
hosts?


If evolutionary arms race favors sex, then
sexually reproducing snails should be
commoner in populations with high rates
of trematode infections.
Results match predictions.
White slice indicates
frequency of males
and thus sexual
reproduction
Frequency of males increases with increasing rates of
trematode infection.
Class Monogenea


The monogenetic flukes were previously
classified as on order of the Trematoda, but
recent work suggests they are more closely
related to cestodes (tapeworms).
Monogeneans are typically external parasites of
fish that clamp onto the gills using a hooked
organ called an opisthaptor. Some also
parasitize frogs and turtles.
Figure 14.16
8.11
Monogenean Fluke
Attached monogeneans
http://www.bogleech.com/flatworms.html
Class Monogenea



Unlike the trematodes, Monogeneans have
only a single host.
The egg hatches into a ciliated larva which
seeks out its host in the water.
Attached to gills, adult feeds on mucus,
epithelium and blood.
Monogeneans attached to gills
of goldfish

https://www.youtube.com/watch?v=OiOez
nXQZuI
Class Cestoda (tapeworms)


Tapeworms are parasites of the vertebrate
digestive tract and about 4000 species are
known.
Almost all tapeworms require at least two
hosts with the definitive host being a
vertebrate, although intermediate hosts
can be invertebrates.
Class Cestoda


Members of the Class Cestoda (tapeworms) are
quite different in appearance from the other
members of the Platyhelminthes.
They have long, flat, tape-like bodies composed
of a scolex for attaching to their host and a
chain of many reproductive units or proglottids
called strobila. New proglottids form behind the
scolex and the strobila may become extremely
long.
Figure 14.18
8.12
Tapeworm scolex
Hooks
Suckers
The scolex is equipped with suckers and hooks that
enable it to grip onto its host’s intestines.
Class Cestoda

Tapeworms live in the intestines and
because they are immersed in digested
food lack a digestive system of their own.
Instead they simply absorb food across
their tegument.
Class Cestoda


To facilitate the absorption of food a
tapeworm’s tegument has huge numbers
of tiny projections called microtriches,
which are broadly similar to the microvilli
of the vertebrate intestine.
They similarly increase the surface area of
the tegument for absorption.
Figure 14.17
8.13
Class Cestoda

Tapeworms are usually monoecious (have both
male and female reproductive organs).

A proglottid is fertilized by another proglottid in
the same or a different strobila.

Shell-encased embryos form in the uterus and
exit the proglottid via a uterine pore or the
entire proglottid may detatch and pass out of
the host.
Figure 14.20
8.14
Human tapeworms

Humans are definitive hosts to several
tapeworms including the beef tapeworm
Taenia saginata, pork tapeworm T. solium,
and fish tapeworm Diphyllobothrium
latum.
Human tapeworms

The lifecycles of these parasites are similar.

Shelled larvae are shed into the environment.

These are consumed by the intermediate host
and the larvae (oncospheres) hatch, bury into
blood vessels and make their way to skeletal
muscle where they encyst becoming so called
“bladder worms” or cysticerci.
Human tapeworms


The encysted larva develops an invaginated
scolex and waits, perhaps for years, for its host
to be eaten.
If the meat is uncooked the cysticercus extends
its scolex, attaches to the wall of the intestine
and within 2-3 weeks matures and begins
growing and producing eggs. A tapeworm may
be many meters long and live for years.
Figure 14.19
8.15
Humans as intermediate hosts


Humans may become intermediate hosts for
tapeworms with potentially disastrous
consequences if they consume shelled larvae in
contaminated food.
In an evolutionarily unfamiliar environment,
cysticerci may encyst in inappropriate locations
such as the brain, which is frequently fatal.
Figure 14.21
Cysticerci in human brain
8.16
Tapeworm manipulations of hosts


Continuing the theme of parasite manipulations
we’ve seen this semester it’s not surprising that
some tapeworms also manipulate their hosts to
ensure they can complete their lifecycles.
A good example is the tape worm Hymenolepis
diminuta, which parasitizes rats and flour
beetles.
Tapeworm manipulations of hosts


Adults live in the bowels of rats (where
they can grow to 45 cm in length) and
produce eggs which pass out of the gut in
rat droppings.
Beetles are attracted to and eat rat
droppings that contain tapeworm eggs by
an apparently highly attractive scent.
Tapeworm manipulations of hosts


Not clear whether the eggs, adult tapeworms, or
rat host produce this scent, but beetles strongly
prefer egg-containing feces to those that are
egg free.
Once in the beetle the tapeworm produces
several chemicals that sterilize female beetles by
blocking the flow of nutrients that allows egg
formation.
Tapeworm manipulations of hosts


In order to reach its final host the
tapeworm needs to ensure that the beetle
gets eaten by the definitive host, a rat.
The tapeworm produces chemicals, which
make the beetle less likely to conceal itself
as well as sluggish and slow to escape if
attacked.
Tapeworm manipulations of hosts



As a final trick the tapeworm also inactivates the
beetle’s last line of defense.
Flour beetles have glands in the abdomen that
spray a foul-tasting liquid, which often will cause
a rat to spit out a beetle it has started to eat.
The tapeworm blocks the gland that makes this
chemical, so that the beetle doesn’t taste bad to
the rat and is consumed.
Parenchymia
Platyhelminthes
Nemertea
Annelida
Mollusca
Sipuncula
Entoprocta
Rotifera
Lophotrochozoa
Acanthocephala
Ectoprocta
Lophophorata
Brachiopoda
Phoronida
Phylum Nemertea (Rhynchocoela)
Ribbonworms


The nemerteans (ribbon worms) are long,
marine, predatory worms and there are
about 1000 species known.
Most are less than 20cm in length, but
others are many meters in length.
Giant Nemertean
• Longest known in phylum
• Up to 30 meters long
• Britain and Ireland
• Shallow waters, pools and
mud
• 10 – 20 eyes
http://farm3.static.flickr.com/2177/2169551899_0667313f6c.jpg
http://www.marlin.ac.uk/images/distribution_maps/uklinlon.jpg
Figure 14.25
8.19
Baseodiscus mexicanus a nemertean from
the Galapagos Islands
Phylum Nemertea (Rhynchocoela)
Ribbonworms


The general body plan of nemerteans is
similar to that of turbellarians.
Like turbellarians they have a ciliated
epidermis and possess a large number of
gland cells. They also have flame cells.
Phylum Nemertea
(Rhynchocoela) Ribbonworms
•
•
Unlike members of the Platyhelminthes
nemerteans have a complete gut with a
mouth and anus
Also possess a true closed circulatory
system. There are blood vessels, but no
heart. Instead valves and muscles control
blood flow.
Phylum Nemertea
(Rhynchocoela) Ribbonworms

The flame cells are associated with the
circulatory system and so are used to
eliminate metabolic wastes (excretion)
rather than just being used for
osmoregulation as in Platyhelminthes.
http://www.dnr.sc.gov/marine/sertc/images/photo%20gallery/nemertean.jpg
Phylum Nemertea (Rhynchocoela)
Ribbonworms



Prey is captured using a long muscular proboscis armed
with a barb called a stylet.
The proboscis lies in a body cavity called the rhynchocoel
and muscular pressure on fluid in the rhynchocoel causes
the proboscis to be quickly everted. The rhynchocoel
appears to be a modified coelom.
The prey is wrapped in the sticky, slime-covered, proboscis
and stabbed repeatedly with the stylet. Neurotoxins in the
slime incapacitate the prey.
Figure 14.24a
Figure 14.24b
8.18
Internal structure of female ribbon worm
(above).
Nemertean with proboscis extended (right)
Phylum Nemertea
(Rhynchocoela) Ribbonworms


Feed on: Small invertebrates
(crustaceans, nematodes, polychaetes,
larvae, etc). Invertebrate eggs.
http://www.youtube.com/watch?v=EwcEY
AGu07E