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Phylum Platyhelminthes
The Flatworms
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Platyhelminthes
26,500 species of free living & parasitic flat worms
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Lophotrochozoa
Annelids
https://ucmp.berkeley.edu/phyla/lophotrochozoa.html
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Platyhelminthes. 2017. Collins, J. J. III
Often speculated in the classic literature to represent
primitive basal bilaterians, modern molecular
phylogenetic analyses place the Platyhelminthes
within the Lophotrochozoa, a clade of invertebrate
animals that includes annelids (segmented worms) and
mollusks.
Current Biology. Cell Press
Volume 27, Issue 7, 3 April 2017, Pages
R252-R256
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Platyhelminthes
All of these are free-living
flatworms
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Platyhelminthes
All of these are parasitic
flatworms
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Subphylum: Rhabditophora
Infraphylum: Trepaxonemata
Euneoophora
Class: Acentrosomata
Trematoda
Monogenea
Cestoda
Bothrioneodermata
Infraclass:
Neodermata
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Phylum Platyhelminthes
Subphylum
Rhabditophora
Subphylum
Catenulidea
Class
Acentrosomata
Cohort
Trematoda
Cohort
Monogenea
Cohort
Cestoda
Superorder
Digenea
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Phylogeny &
Taxonomy
Phylum Platyhelminthes
Subphylum Catenulidea
Subphylum Rhabditophora
Class Acentrosomata
Cohort Trematoda
Cohort Monogenea
Cohort Cestoda
Superorder Digenea
Class Acentrosomata — lack genes controlling
centrosome formation
Cohort Trematoda — liver, & blood flukes
Superorder Digenea — Digenean flukes
Cohort Monogenea — Monogenean flukes
Cohort Cestoda — Tapeworms
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Phylogeny &
Taxonomy
Phylum Platyhelminthes
Subphylum Catenulidea
Subphylum Rhabditophora
Class Acentrosomata
Cohort Trematoda
Cohort Monogenea
Cohort Cestoda
Superorder Digenea
Class Acentrosomata — lack genes controlling centrosome formation
Cohort Trematoda
Infraclass Neodermata
Superorder Digenea
Cohort Monogenea
Cohort Cestoda
o Monophyletic group
of all exclusively parasitic
Platyhelminthes
Body Plan
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Body Wall – Free-living
Epidermis
Parenchyme/
mesenchyme
Circular muscles
Gut
Longitudinal
muscles
Dorsoventral
muscles
Cilia
Biology of the Invertebrates 7e, Fig 8.4
Ducts of glands Rhabdoids
Basement membrane
Circular muscles
Diagonal
muscles
Dorsoventral
muscles
Longitudinal muscles
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Maintain high Surface
Area:Volume ratio
Body Plan
Pros
Pseudogeoplana reticulata
diffusion works well
Cons
diffusion works too well
ionic balance
Luteostriata ernesti
osmoregulation
desiccation
Rhynchodemus sylvaticus
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Parasitic Platyhelminthes, Flukes
Oral sucker
Ganglion
Intestine
Prohaptor
Excretory
duct
Opisthaptor
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Scolex
Sucker
Scolex
Cestodes
Proglottids
Strobila
Parasitic
Platyhelminthes:
Cestoda
(Tapeworms)
Lateral
nerve cord
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Parasitic
Platyhelminthes
Tegument
(body wall)
of parasitic
flatworms
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Support, Locomotion
and Attachment
Free-living forms
Figure 8.4
cilia-powered
gliding and
muscular
undulations
Biology of the Invertebrates 7e, Fig 8.4
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Support, Locomotion and Attachment
Free-living forms
cilia-powered gliding
and muscular
undulations
Ventral cilia
Pedal waves
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Support, Locomotion and Attachment
Biology of the Invertebrates 7e, Fig 8.6
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Support, Locomotion,
and Attachment
Parasitic forms
attachment structures
—hooks, adhesive organs, scolices—
Digenean Fluke
Oral sucker
Cestode
Monogenean Fluke
Scolex
Buccal sucker
(prohaptor)
Acetabulum
Opisthaptor
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Attachment organs of
monogenean flukes
Prohaptor (anterior)
Opisthaptors (posterior)
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Attachment organs of cestodes
{
Scolex
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Feeding and Digestion
Free-living
flatworms
Biology of the Invertebrates 7e, Fig 8.9B
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Feeding and Digestion
Free-living flatworms
Sacklike
gut
Tribranched
gut
Multibranched
gut
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Feeding and Digestion
Free-living flatworms
Muscular proboscis in Gnathorhynchus (left) &
Cheliplana (right)—unique among flatworms
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Feeding and Digestion
Free-living flatworms
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Feeding and Digestion
Parasitic flatworms
Pharynx
Intestine
Monogenean fluke
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Circulation and Gas Exchange
No specialized organs
Lack circulatory system
some have hemoglobin
GV cavity & diffusion
Endoparasites capable of
anaerobic metabolism
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Figure 8.4
Excretion and Osmoregulation
Protonephridia
Biology of the Invertebrates 7e, Fig 8.4
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Excretion and Osmoregulation
Protonephridia are found in nearly all flatworms
Protonephridial
network
Ciliary
flame
Flame
bulb
Nephridiopores
to nephridiopore
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Excretion and Osmoregulation
Protonephridia are found in nearly all flatworms
Protonephridial ducts
in a cestode proglottid
Ventrolateral
longitudinal
excretory ducts
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Nervous System and Sense Organs
Ladder-like nervous system: free-living flatworms
cerebral
ganglion
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Nervous System and Sense Organs
Auricle
Comissures
Ocellus
Optic
nerve
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Nervous System and Sense Organs
Parasitic flatworms
Trematode
Cestode
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Reproduction & Development
Free-living flatworms—asexual reproduction
Fig. 1: Pluripotent stem cells enable planarians
to achieve extraordinary feats of regeneration
From: Bartscherer, K. 2014. Flatworms, the masters of regeneration
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Reproduction & Development
Free-living flatworms—sexual reproduction
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Reproduction
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Reproduction & Development
Parasitic flatworms—sexual reproduction
Fasciola hepatica,
a digenean fluke
larva of a
monogenean fluke
Swims until appropriate host located
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Reproduction and Development
Example of monogenean fluke re-infecting host
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Reproduction and
Development
Example of digenean
fluke life cycle:
human blood fluke
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Reproduction & Development
Parasitic flatworms: sexual reproduction
Cestodes
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Parasitic Helminths &
Human Health
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Parasitic Helminths & Human Health
When urban living reduced human contacts with
microbes and worms, it increased our risk for
chronic inflammatory disorders
Microorganisms and macroorganisms (i.e., helminths from mud, animals,
and feces) play a critical role in driving immunoregulation.
The term "old friends" implicates exposures to microbes and other
organisms during critical phases of human development.
Graham A. W. Rook 2011. Microbe Magazine
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Common Autoimmune Disorders
Thyroid
Digestive system
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Hashimoto’s Thyroiditis
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Celiac disease
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Grave’s Disease
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Crohn’s disease
Blood
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Joints
Pernicious anemia (B12)
Liver
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Rheumatoid arthritis
Hair
Autoimmune hepatitis
Pancreas
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Alopecia areata
Nervous system
Type 1 diabetes
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Multiple sclerosis
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Prevalence of Infectious vs Autoimmune Diseases
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Parasitic Helminths
Diseases and conditions of the modern era
e.g., multiple sclerosis, type-1 (and -2) diabetes, and allergies
involve disrupted immunoregulatory circuits, likely reflecting
reduced exposures to "old friend" organisms with which
humans coevolved.
Several clinical trials are testing these concepts,
Can renewed exposures to "old friend" organisms help to
combat these modern-era diseases?
Graham A. W. Rook 2011. Microbe Magazine
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“[There is a] reciprocal distribution between parasitic diseases in
many of the world’s lower-income countries, and ever-increasing
levels of inflammatory disorders such as allergy, autoimmunity,
and inflammatory bowel diseases in the more affluent
societies.”
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