Download Invertebrate Zoology Lecture 2, March 31, 1999

Biology 212 Lecture 3:
Phylum Platyhelminthes, Phylum Nematoda, Phylum Annelida
January 11, 2008
I.
Phylum Platyhelminthes (flatworms)
A.
Overview
B.
Evolutionary/Morphological Relationships
1.
Diverge from the Cnidaria in the following ways:
a)
Triploblastic: three embryonic germ layers

What are they?

Role of mesoderm?

Body cavity present?
b)
Bilateral symmetry (Bilateria)

Accompanied by cephalization (what is this?)
2.
Belong to the branch of animals known as Protostomes
a)
Differences between Protostomes and Deuterostomes [See Fig. 32.9]

Describe four key differences
C.
Body organization [Fig. 32.8, 33.10]
1.
Eversible pharynx
a)
Functions? Where is the mouth?
2.
Gastrovascular cavity
a)
Structure

How does it differ from the gastrovascular cavity of Cnidarians?

Is this a complete digestive system?
b)
Functions?
3.
Water/ion balance and ammonia excretion [Fig. 44.10]
a)
Have protonephridia, very simple excretory system (a.k.a. “flame bulb
system”)

Tuft of cilia moves, creates a negative pressure to draw excess water
into a system of tubules and out through pores
b)
Plays minimal role in movement of nitrogenous waste (i.e. ammonia)
4.
Nervous system
a)
Brain?
b)
Nerve cords?
c)
Sensory structures?
5.
Movement
a)
Cilia/mucus
b)
b) muscles
6.
Reproduction
a)
Hermaphrodites
b)
Internal fertilization

penis, sometimes with stylet


hypodermic impregnation
penis also used to...???
Other aspects
a)
Many parasitic forms

Example: tapeworms [Fig. 33.12]
Phylum Nematoda [Fig. 33.26]
7.
II.
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A.
II.
Basic appearance/diversity
1.
covered with cuticle
a)
It molts this as it grows, just like arthropods.

Sometimes grouped with Arthropoda; connection supported by
molecular evidence.
2.
More nematodes than members of any other animal phyla
3.
Found in nearly every environment
4.
Free-living, but also important parasites on plants/animals
B.
Evolutionary relationships
1.
Same as for Platyhelminthes (Eumetazoa, Bilateria, Protostomes)
C.
Have a body cavity [Fig. 32.8
1.
In the nematode, it is a "pseudocoelom", not a true coelom.
a)
Why called a pseudocoelom rather than a true coelom (a.k.a. “eucoelom”)?
2.
Functions of pseudocoelom?
3.
NOTE: Cannot move its gut independently of its body wall
D.
Feeding/digestive system:
1.
Muscular pharynx lined with cuticle
a)
Powerful suction can be created by change in pharynx shape/size ("Nematodes
suck")
2.
Some have stylets (both parasites, predators)
3.
Relatively simple (not highly branched) gut, but it is complete, with anus.
a)
What’s so great about an anus?
E.
Gas exchange:
1.
Body surface
2.
Coelomic cavity
3.
NOTE: No circulatory system
F.
Excretory system/osmotic regulation
1.
Exists, not well-understood ("lateral line" in dissection)
G.
Locomotion:
1.
Have only longitudinal muscles. How does this limit movement?
2.
Cuticle is also elastic, helps in motion (springiness)
H.
Nervous system
a)
Brain?
b)
Nerve cord(s)? Location?
c)
Ganglia? Location? And what are ganglia, anyway?
2.
NOTE: Connection of longitudinal muscles is unique: extensions of muscle cells
attach to nerves, instead of the other way around.
3.
Sensory: several simple structures for detected light, chemicals, pressure, etc…
I.
Reproduction
1.
sexes usually separate, but some hermaphrodites
2.
internal fertilization
J.
Parasites (examples)
Phylum Annelida: the segmented worms
A.
Where Annelids fit in evolutionary scheme
1.
Protostome
2.
Have a true coelom (eucoelom) completely lined with mesoderm
a)
See chart for comparison of pseudocoelom with eucoelom.
B.
Have body segmentation, with similar structures found in each segment.
1.
Advantages?
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C.
D.
E.
a)
Increased surface area
b)
Redundancy of functions
2.
Some specialization of regions (examples)
Three classes
1.
Class Polychaeta: marine worms
2.
Class Oligochaeta: earthworms
3.
Class Hirudinea: leeches
Body plan: Earthworm focus (with some notes on others)
1.
Digestive system
a)
Complete, allowing for…?
b)
Specialized regions (examples)
2.
Circulation
a)
Closed system: blood contained in vessels
b)
Multiple hearts
3.
Gas exchange
a)
Body surface
b)
Coelom
c)
Parapodia (Class Polychaeta only)
4.
Excretory
a)
Metanephridia

Kidney-like, think about terrestrial lifestyle of earthworms
5.
Nervous system
a)
Brain?
b)
Nerve cord(s)? Location?
c)
Ganglia? Location?
6.
Movement
a)
Peristalsis: Possible only with both circular and longitudinal muscle.
b)
Setae
c)
Some Polychaetes have leg-like parapodia
Basics of feeding in diff. classes
1.
Class Oligochaeta: Earthworms
a)
Earth suckers (create vacuum by expanding muscular pharynx); lots left
undigested
b)
Process 40 tons of soil/acre of earth/year!
c)
improve aeration/drainage
2.
Class Polychaeta: Marine segmented worms
a)
Many are tentacle feeders
b)
Many other feeding modes
3.
Class Hirudinea: Leeches
a)
Not all are bloodsuckers—others are predators and scavengers.
b)
Create vacuum by expanding pharynx, digestive tract
c)
Blood suckers hang on with suckers, pierce with jaws or break through
w/enzymes, have a pharynx they stick into prey, suck out blood.
d)
Blood suckers secrete anticoagulants, vasodilators and numbing agents
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