PLATYHELMINTHES
• Eumetazoa: Bilateria: Phylum
Platyhelminthes (flatworms)
• marine, freshwater, terrestrial
PLATYHELMINTHES
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either Protostomia or Lophotrochozoa
triploblastic: 3 germ layers
ectoderm: epidermis, nervous tissue
mesoderm: true muscle tissue
endoderm: gastrodermis lining
gastrovascular cavity
• acoelomate
PLATYHELMINTHES
• lack some organ systems (circulatory,
respiratory)
• diffusion: reason why small and flat
• have others (muscular, nervous,
digestive, excretory, reproductive)
• nervous system (Fig. 49.2): eye spots,
rudimentary brain, nerve cords
• cephalization (head); Fig. 33.10, Fig.
S4-2
PLATYHELMINTHES
• hermaphroditic: male and female
reprod. systems in same individual
• adaptive advantage: every individual
potential mate
• digestive system: mouth, pharynx,
gastrovascular cavity
• one way gut; no anus
• invagination: increases surface area
PLATYHELMINTHES
• excretory system: protonephridia (Fig.
44.11); proto = first, nephrus = kidney
• branching network of tubules
• flame-bulb cell with cilia draws mostly
water, some waste into tubules
• exit via nephridiopores
PLATYHELMINTHES
• free-living (non-parasitic) ancestral life
history
• many parasitic in vertebrate gut
• suckers/hooks; Fig. S4-3
• reduce/lose some ancestral characters
• tapeworms: no mouth/gastrovascular
cavity/excretory structures; diffusion
• mostly reproductive organs
• Fig. 33.12
MALFORMED FROGS
• mystery of the malformed frogs
• August 1995: school kids in MN, field
trip to a pond; amphibian malformation
MALFORMED FROGS
• most states, range of deformities
• hypothesis: endocrine disrupting
chemicals
• disrupt development by disrupting
hormones
MALFORMED FROGS
• alternative: trematode hypothesis
• trematodes (= flukes)
• Ribeiroia life cycle: encysts in limb buds
MALFORMED FROGS
• can cause deformities, but why now?
• human-caused ecosystem disruption
• nutrient runoff: organic pollution
(fertilizer, manure, sewage)
• increased snail population (a key host)
• modified hypothesis: nutrient-trematode
NEMATODA
• Eumetazoa: Bilateria: Phylum
Nematoda (roundworms)
NEMATODA
• Protostomia or Ecdysozoa
• pseudocoelom
• complete digestive tract (2 openings)
hangs free in pseudocoelom; Fig. S5-1
• cylindrical, unsegmented body
• cuticle: tough, thick outermost layer
around epidermis; ecdysis
NEMATODA
• hydrostatic skeleton: fluid in
pseudocoelom; high pressure
• nervous system: anterior nerve ring,
dorsal/ventral nerve cords
• no circulatory/respiratory systems;
diffusion
• muscular system: longitudinal muscle
• locomotion: contraction; thrashing
NEMATODA
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only sexual reproduction (Fig. S5-2)
some hermaphroditic, others dioecious
dioecious: separate male, female
reprod. system fills most of organism
NEMATODA
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free-living (soil); parasitic
uniform structurally, diverse ecologically
species differ in specific habitat
probably over one million species:
speciose
• 1 sq m of marine mud: 4.5 million
individuals!; very successful
• much to be learned
NEMATODA
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Caenorhabditis elegans (C. elegans)
model organism
free-living soil nematode; easy to raise
transparent; life cycle: 3½ days
every cell mapped, fate known; Fig.
47.22
C. elegans
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entire genetic sequence known
huge medical applications
humans, nematodes both bilaterians
evolutionary medicine: studying other
organisms for insight into humans
ANNELIDA
• Eumetazoa: Bilateria: Phylum Annelida
(segmented worms)
• Protostomia or Lophotrochozoa
• coelomate
• segmentation: repeated muscular,
nervous, excretory elements divided by
septae
• annelida = little rings
• Fig. 33.22
ANNELIDA
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hydrostatic skeleton: fluid in coelom
circulatory system: closed; Fig. S6-3
heart: muscular blood vessels
dorsal, ventral blood vessel; Fig. S6-1
capillaries: small branches of blood
vessels
ANNELIDA
• excretory system: metanephridia (Fig.
44.12)
• fluid enters nephrostome, passes
through coiled tubule
• useful nutrients return to circ. system,
wastes excreted via nephridiopore
ANNELIDA
• nervous system: ventral nerve cord with
ganglia (swellings); brain
• muscular system: longitudinal and circular
muscles
• locomotion: muscular contraction works
against hydrostatic skeleton of coelom
• setae (bristles)
• digestive system: subdivided and more
specialized
• hermaphroditic or dioecious
ANNELIDA
• leeches
• specialized as predators or blood
feeders
• don’t confuse with trematode
• no obvious head; no septae or setae
• suckers for temporary attachment
• large digestive system for blood
• secrete hirudin: anticoagulant
• other secretions; osteoarthritis