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Bi 212: Lecture 3
Platyhelminthes, Nematoda,
Annelida
Phylum Platyhelminthes:
The flatworms
 Require an
aquatic habitat
 Often under rocks
 Some freeliving, but many
parasitic forms
 Hosts provide
aqueous
environment
Evolutionary relationships
Platyhelminthes are triploblastic,
but have no body cavity
 Note positions of embryonic tissue
layers (section is not through
pharynx…)
Protostomes vs. deuterostomes
Basic Platyhelminthes features
Eversible pharynx
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How does it work?
Gastrovascular cavity
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Functions
Water and ion balance
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Protonephridia (flame bulb
system)
Nervous system
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Brain? Nerve cords?
Sensory structures?
Movement
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Muscles
Cilia and mucus
Reproduction
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Hermaphrodites
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Internal fertilization
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penis, sometimes with stylet
hypodermic impregnation (R-rated flick)
Penis is also used for???
 Tapeworms are one many
types of parasitic
Platyhelminthes
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 33.12
Phylum Nematoda:
The roundworms
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Covered with cuticle like
Arthropoda
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Contains chitin
Molts cuticle as it grows
Most abundant phylum in
terms of numbers (?)
Found in nearly every
environment
Some free-living, but also
important parasites on
plants/animals
Evolutionary relationships
Nematode body cavity:
pseudocoelom
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Mesoderm on inside of body wall, but not
surrounding gut (thus not a true coelom or
“eucoelom”)
Roles of pseudocoelom?
Body plan
Feeding/digestive system
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Nematodes suck!
Complete digestive system
Gas exchange
Body surface
Intestinal surface (minimal)
Body cavity (pseudocoelom)
No circulatory system
Excretory system/osmotic regulation
“lateral line”, not well understood
Locomotion:
Have only longitudinal muscles
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Demo: The nematode swim!
Elastic cuticle
Body plan (cont.)
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Nervous system
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“Brain”: nerve ring
Four nerve cords (where?)
Ganglia along ventral cord
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Unique connection of muscles and nerves
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What are ganglia?
What is unique about them?
Sensory structures
Reproduction, etc…
Reproduction
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Sexes usually separate, but some hermaphrodites
Internal fertilization
Many parasitic forms
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Trichina worms
Heartworms
River blindness
Elephantiasis
Also parasitic in plants
Phylum Annelida:
The segmented worms
Evolutionary relationships
Acoelomates, Pseudocoelomates and
Coelomates
Pseudocoelom
 Surface area for
nutrient absorption
and gas exchange
Coelom
+
+
Larger organs
+
Fluid cushions/
+
protects organs
Hydrostatic skeleton +
is possible
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+
Completely lined by mesoderm
Mesenteries suspend internal organs
Independent
movement of gut
+
+
+
+
Basic Annelida features
 Have all features of protostomes
(know!)
 Have body segmentation (new
feature)
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Repetition of body parts
Similar structures found in each region
Some specialization of regions
Some continuous parts (digestive system)
Annelid diversity
Class Polychaeta
Class Hirudinea
Class Oligochaeta
Body Plan
 Digestive system
 Complete
 Specialized regions
 Circulation
 Closed system
 Multiple hearts
 Gas exchange
 Body surface
 Coelom
 *Parapodia
(Polychaetes only)
 Excretory
 Metanephridia
Body Plan (cont.)
 Nervous system
 Brain (=cerebral
ganglia)
 Ventral nerve cord
(fused pair of cords)
 One ganglion per
segment
 Movement
 Peristalsis
 Setae
Feeding in different classes
Class Oligochaeta: Earthworms
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Earth suckers (create vacuum by expanding muscular
pharynx)
Why important in ecosystem?
Class Polychaeta: Marine segmented
worms
Many are tentacle feeders
Many other feeding modes
Class Hirudinea: Leeches
Food suckers (create vacuum by expanding pharynx,
digestive tract)
How do blood sucking leeches feed?.
What three substances do blood sucking leeches
secrete to aid in the process?
NOTE: Not all leeches are blood suckers!