PHYLUM PLATYHELMINTHES
The Acoelomates
ZLY 103
Animal Diversity
Introduction
• The first triploblastic animals with a third tissue
layer, mesoderm; between the endoderm and
endoderm.
• Due to absence of a coelom or pseudocoel, they are
termed acoelomate
• They are flat in shape and possess a blind gut (mouth
without anus).
• Size varies from a millimeter to several meters.
• Some are free-living (Turbellaria) and parasitic.
• Majority are bottom-dwellers in aquatic & moist
lands.
Introduction
• Eumetazoa - animals with tissue
– Bilateria - have bilateral symmetry and are
triploblastic.
– Protostomia - a group of animals whose mouth
develops from the blastopore, and the
mesoderm forms from an area near the
blastopore.
– Acoelomata - They have a true mesoderm
which fills the original blastocoel between the
outer epidermis and digestive tract.
Characteristics
• They are triploblastic, acoelomate, non metamerically
segmented metazoans.
• They are at organ system level of organization.
• They exhibit bilateral symmetry,
• Body contains no internal cavity and absence of blood
system.
• Protonephridial excretory organ (Flame cells) instead
of anus.
• Nervous system made up of longitudinal fibers
instead of a nerve net.
• Dorsoventrally flattened.
Characteristics
• They carry out hermaphroditic reproduction.
• Free-living forms feed on small animals and other life
forms.
• Parasitic forms feed on nutrients derived from the
host tissues.
• Some species especially the free-living occur in major
biomes; marine, freshwater & moist lands.
• Central nervous system consists of a brain and two or
more ganglionated nerve cord.
• They include tape worms, flatworms, flukes and
planarians etc.
• Highly successful, with about 25,000 species.
Characteristics
• Shows a degree of cephalization
• Epidermis soft and ciliated or covered by cuticle and
with external suckers or hooks or both
• Simple sense organs with eyespot in some species.
• Parasitic species normally move between different
habitats during their developmental stages.
• A few species are commensals deriving harmonious
existence with another larger organism.
• Absence of fossil records due to their soft bodies but
they are thought to have evolved around 550my ago.
Triploblastic Condition
• The embryo divides into three instead of two as it is
in cnidarians (diploblastic).
• With the presence of a (parenchyma) mesoderm,
the platyhelminthes:
Have an increase in size,
A separation of the alimentary canal from the body
wall,
A variety of organs are now formed,
There is a movement towards the organ-system level
of organization,
Presence of developing muscles, Cilia and flagella are
becoming too irrelevant for the larger sizes evolving.
The Acoelomate Condition
• The large size of platyhelminthes with the
emergence of the mesoderm (next slide) created the
problems of efficient transport system, hence the
acoelomate condition.
• The animals in this group do not have a choice but
to be dorsoventrally flattened for it to present a
large surface area for diffusion to satisfy the
metabolic requirements (flatworms).
Internal section through the gut
Plate 1: Transverse section through a generalized Acoelomate
Epidermis/
ECTODERM
gut cavity
Lining of gut
=ENDODERM
Acoelomate organism
(flatworm)
no body cavity; region between
gut and body wall packed with
organs within MESODERM
tissue
Body plan
Body design in Platyhelminthes
Taxonomy/Classification
• Class Turbellaria
• Dugesia
• Class Monogenea
– Gyrodactylus
• Class Trematoda
– Clonorchis, Fasciola, Schistosoma
• Class Cestoda
– Taenia, Dipylidium, Echinococcus
Distinct Features of the Classes in Flatworms
• Proglotids and scolex in Cestoda
• Rabdites in Turbellaria
• Combination of characteristics- protonephridia for
excretion and acoelomates in all except Cestodes
• Larva of each class are unique
– Turbellaria - Planaria
– Cestodes - Oncosphere/Cysticerci (bladder worm)
– Trematodes - Metacercaria/Cerceria
Distinct Features of the Classes in Flatworms
• Integuments- Rhabdites and one cell layer
epidermis in Turbellaria and usually ciliated;
syncytial tegument in other classes.
• Skeletal - hydrostatic
• Muscle - longitudinal, transverse, and circular
muscles are present.
• Digestive - incomplete with intracellular and
extracellular digestion; no system in Cestoda.
Distinct Features of the Classes in Flatworms
• Excretory - flame cells, or excretory tubes in
Cestodes.
• Respiratory - no system, through diffusion
• Circulatory - none, diffusion.
• Nervous - anterior ganglia, ventral ladder-like
system (two lateral cords with transverse
cords).
Distinct Features of the Classes in Flatworms
• Endocrine - hormones produced by nervous
system
• Reproductive - monoecious in most
– Well developed reproductive organs, mostly
internal fertilization.
– Two of the parasitic classes have complex life
cycles
• Trematoda - miracidium, sporocyst, redia,
cercaria, metacercaria (usually snail is
secondary host.
• Cestoda - oncosphere, cysticercus
Distinguishing Features among the Classes
Turbellaria
Free living;
aquatic
Delicate, soft
body
Suckers rarely
present
Trematoda
Endoparasitic
Monogenea
Ectoparasitic,
larva is freeliving
Leaf-like shape leaflike/cylindrical
in shape
Ventral sucker
plus sucker on
‘head’ for
attachment to
host
Enteron present Enteron present
posterior
attachment with
hooks, suckers
or clamps
Cestoda
Endoparasitic
or ectoparasitic
Elongated,
proglottides
(able to detarch)
Suckers and
hooks on ‘head’
plus sucker on
‘scolex for
attachment
Enteron absent No enteron
Turbellaria
Trematoda
Cuticle absent Thick cuticle
(outer surface with spines (not
covered with
parasitic)
cilia)
Hermaphroditic/ Indirect
asexual fusion development
Sense organs in Sense organs
adult
only in freeliving larval
stages
e.g. Planaria,
e.g. Fasciola
Dugesia,
(liver fluke)
Planocera,
Schistosoma
Microstomum (blood fluke)
Monogenea
Cestoda
Syncytial
teguments, no
cilia
Thick cuticle
(protection); no
cilia in adult
Direct
development
Sense organs
only in freeliving larval
stages
e.g.
Indirect
development
Sense organs
only in freeliving larval
stages
e.g. Taenia
(tapeworm)
Dactylogyrus,
Polystoma,
Gyrodactylus
Reproduction in Platyhelminthes
Asexual by budding or transverse fission
•Regeneration
Sexual
– Hermaphroditic; complex reproductive
system
– Internal fertilization
– Direct development, but some polyclads have
free-swimming larvae
Reproductive system of Platyhelminthes
Class Turbellaria
• Free-living flatworms, mainly carnivorous.
• Mostly aquatic, a few terrestrial.
• Benthic (bottom dwellers).
• Variable in shape, size and features.
• Best example of cephalization.
• Surprising level of internal complexity
Class Turbellaria
• They are characterized by a soft epidermis
ciliated at on the ventral surface.
• The movement of the cilia propels the smaller
forms.
• Larger species glide along by muscular waves,
usually over mucous beds secreted by special
cells.
Turbellarians
• Generally divided into five groups based primarily
on differences in the form of the digestive cavity.
• The most primitive are the acoels, (no digestive
cavity).
• The ventral mouth, and sometimes a simple
pharynx, lead to an inner mass of nutritive cells.
• Most species measure less than 1/8 in. (3 mm) in
length.
Planaria: Tubellaria
Turbellarians
• The rhabdocoels have straight, unbranched
digestive cavities.
• Studies show that the rhabdocoels gave rise to
both the trematodes and cestodes,
• Several rhabdocoel species exhibit commensal
relationships, presumably led to parasitism.
• The allocoels were formerly classified together
with the rhabdocoels; the gut can be either
saclike or branched.
Diversity in Planarians
• The triclads, also known as planarians, are
relatively large flatworms named for their
three-branched gut (NEXT SLIDE).
• Most species range from 1/8 in. (3 mm) to
about 1 in. (2.5 cm) in length.
• Planarians have more sense organs and a
more complex brain than the other
turbellarians (Polyclads).
Diversity in intestinal pattern of two orders of
Tubellarians
Tricladida
Polycladida
Biology of Turbellaria
• The freshwater species Dugesia tigrina has
primitive eyes and tactile lobes, or auricles, on
the sides of the head.
• Muscular pharynx can be extruded for food
capture.
• Dugesia and many other planarians can
regenerate entirely new individuals from small
pieces cut from the body.
Biology of Turbellaria
• The group of turbellarians known as polyclads
tend to be larger (1–2 in./2.5–5 cm) and more
oval-shaped than the triclads.
• Their bodies are extremely flat and leaf-like, and
the gut is subdivided into numerous branches.
• Many are brightly colored and some have
ruffled edges. Some species have numerous eyes
scattered over the front end of the body.
Class Trematoda
• Flukes - oral and ventral suckers, no hooks,
parasites, body with a syncytial tegument without
cilia. Larva stages unique.
• Clonorchis
• Fasciola
• Schistosoma
Class Trematoda
E.g. Flukes
• Most important is subclass Digenea
• 11,000 species; second most diverse
group of parasites (Nematoda#1)
• Endoparasites of vertebrates
• Some cause debilitating diseases to
livestock and humans
Types of Hosts
 Often have complex life cycles that alternate
between sexual and asexual stages.
 Most require at least 2 different kinds of hosts to
complete their life cycle:
1. Definitive host (primary host)
 Host in which the parasite matures and reproduces
(sexually)
 Host in which eggs are released
2. Intermediate host
 Hosts in which larval stages develop and undergo
asexual reproduction
 Results in increase in the number of the individuals
Clonorchis
• Human liver fluke - has two intermediate or
secondary host snail and fish.
Adult human liver fluke
Cerceria
Clonorchis Life Cycle
8.8
Fasciola
Adult sheep liver fluke
metacercaria
on grass.
Fasciola Body Plan
Life cycle of Fasciola
Schistosoma
• Blood Flukes
 Cerceria usually infect by burrowing through skin.
Schistosomiasis




Schistosomiasis (bilharzia) is an infection with
blood flukes and is a major infectious diseases.
More then 200 million people are infected
worldwide with these flukes.
They acquire infection by swimming or walking
in water domiciled by the intermediate snail host.
Schistosome eggs enter the water when infected
people urinate or defecate in or near water.
Schistosomiasis




Eggs hatch and the miracidium seeks out a snail.
Within the snail, it develops into a sporocyst and
asexual reproduction takes place. Cercaria are
eventually released into the water.
Cercaria swims and avoids UV light which can
damage it, but is very sensitive to the scent of
humans.
Certain molecules from human skin makes
cercaria jerks and releases chemicals that soften
the skin and burrows in shedding its tail at the
same time (Schistosomula).
Schistosome Life
Cycle
8.9a and b
Class Monogenea



They were previously classified as an order of the
Trematoda, but recent work suggests they are more
closely related to cestodes (tapeworms).
They are small (usually < 2cm) typically external
parasites of fish that clamp onto the gills using a
hooked organ (often with suckers) called an
opisthaptor.
Some also parasitize the urinary bladder or rectum
of frogs and turtles; there is a species that
parasitizes squid and one that attaches to the
eyeball of hippopotomuses.
Class Monogenea



Unlike the trematodes, Monogeneans have only a single host
(hence “Mono” in the name). Most feed on the host’s
epidermis using their protrusible pharynx, but some are blood
feeders.
Monogeneans are hermaphrodites (male organs develop
first) and can move around a host in search of a mate (they
will also self-fertilize).
The egg hatches into a ciliated larva (an oncomiracidium)
which seeks out its host in the water.
Monogenea Body Plan
Gyrodactylus olsoni
Monogenean Fluke on gill of longjaw
mudsucker fish
8.11
Class Cestoda (Tapeworms)
• Have distinct head structure
(Scolex) and reproductive
units
called
proglottids
(Strobila).
• Absence of a digestive system.
• Larva are oncospheres and
cysticerci (bladder worms)
 Taenia
 Dipylidium
 Echinococcus
Class Cestoda (tapeworms)



They 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.
They are quite different in appearance from the
other members of the Platyhelminthes.
Cestodes Proglottids (Body Plan)
& Head




Class Cestoda
New proglottids form behind the scolex and the
strobila may become extremely long.
Tapeworms live in the intestines where it is
immersed in digested food therefore lack a
digestive system. Instead they simply absorb food
across their tegument.
To facilitate the absorption of food, the tegument
has huge numbers of tiny projections called
microtriches.
The surface area of the tegument for absorption is
greatly increased.
Cestode: Taenia
solium
8.12
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 detach and pass out of the host.
Cestode: Life
Cycle
8.15
Dipylidium (dog tape)
• Juveniles in flea and
louse
• Note the two gential
(red arrows) pores and
reproductive systems in
each proglottid.
Proglottid
Echinococcus
• Hydatid cyst (cysticercus)- many scolices and enlarges to
the size of a basketball.
• Often called sand.
• Only three proglottides
Constraints Associated with Parasitic Living
•
•
•
•
•
•
•
•
•
Reproduce within the definitive host
Get fertilized eggs out of the host
Contact a new and appropriate host
Obtain entrance into the host
Locate the appropriate environment within host
Maintain position within the host
Withstand an often anaerobic environment
Avoid digestion or attack by the hosts immune system
Avoid killing the host, at least until reproduction is
completed
End of Presentation
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