Download Lecture 7 The coelom, worms and molluscs

Protostomes 1
Cavities and worms
Tom Hartman
www.tuatara9.co.uk
www.tuatara9.co.uk
Animal form and function 1
Module
Module 11112
11112
New levels of complexity
The main animal branches.
1. Cleavage
a) Spiral, determinate→ protostome
b) Radial, indeterminate → deuterostome
2. Fate of the blastopore
a) Becomes the mouth → protostome
b) Becomes the anus → deuterostome
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Ra
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Pr
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Lophotrochozoa
Ecdysozoa
Deuterostomia
Radiata
3. Additional cell layer: the mesoderm.
4. Cavity within the body.
Parazoa
Where on the tree?
Ecdysozoa
Lophotrochozoa
Protostomes
• Lophophore: a horseshoe shaped crown of
ciliated tentacles used for feeding.
• Trochophore: a type of larvae (found in
many of the phyla despite widely different
adult forms).
Insects, cruscatea, myriapods, arachnids
Velvet worms
Round worms
Lophotrochozoa
Strange bivalves
Snails, octopi, bivalves, etc.
Triploblastic
Bilateralia
Eumetazoa
Deuterostomes
Segmented worms
Flat worms
Wheel mouths
Lanclets, vertebrates, etc.
Starfish, urchins, etc.
Apical tuft
of cilia
Comb jellies
Radiata
Diploblastic
Mouth
Anus
Parazoa
Anemones, hydras, etc.
Sponges
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Flat worms
Phylum Platyhelmithes
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2.
3.
4.
5.
6.
7.
8.
Big changes from all animals branching earlier.
Bilaterally symmetrical.
Triploblastic in development.
SuperTissues and organs.
phylum
Definite anterior end with cephalisation. Bilateralia
Simple nervous system and brain.
Many have eyes.
Muscular system/cilia for movement.
Free living and parasitic.
Platyhelminthes
• Turbellarians
• Flukes (trematodes)
– parasitic
• Tapeworms (cestodes)
– parasitic
A planarian in cross section
• No circulatory system therefore:
• Flat to cope with O2/CO2
diffusion.
• Sac like or branched gut.
Body covering
(from ectoderm)
Acoelomate. Acoelomates such as
flatworms lack a body cavity between
the digestive tract and outer body wall.
– Gut delivers nutrients in absence of
circulatory system.
– Gut absent in some parasitic forms
(a derived feature).
Tissuefilled region
(from
mesoderm)
Digestive tract
(from endoderm)
• A solid body: no cavities.
• Excretory/osmoregularity organs
(protonephridia).
A solid body with no spaces or cavities: ACOELOMATE.
Parasites can have an extraordinarily
complex life cycle.
Reproduction
• Mostly hermaphrodite with complex reproductive systems
with some species reliant on asexual reproduction: they
tear in half!
• This has lead to some vital observations on the nature of
development and chemical gradients.
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Regeneration in Planaria
a model for developmental biology.
"
Ecdysozoa
Insects, cruscatea, myriapods, arachnids
Protostomes
Velvet worms
Round worms
Lophotrochozoa
Strange bivalves
Snails, octopi, bivalves, etc.
Triploblastic
Bilateralia
Eumetazoa
Deuterostomes
Segmented worms
Plathelminthes
Wheel mouths
Lanclets, vertebrates, etc.
Starfish, urchins, etc.
Ctenophora
Radiata
Diploblastic
Parazoa
Cnidaria
Porifera
Phylum Rotifera
The wheel mouths
– A crown of cilia (look like wheels)
– Mouth with jaws, pharynx and anus
opening into a cloaca with the
reproductive system.
– They have a body cavity derived from
blastocoel (pseudocoelomate).
– Tissues may be syncitial (multinucleate).
– No circulatory system.
– No respiratory organs.
– Good at anabiosis (dry out to survive
inclement conditions).
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Bdelloid rotifers
Scandal in evolutionary biology.
• These rotifers gave up sex 80 mya.
• They are anciently, universally and successfully
asexual.
• No other animal has done this. The switch to
compulsory asexuality normally ends in swift
extinction as variability drops and they cannot
track the environment or parasites.
• This is the scandal of the bdelloid rotifers.
• No males have ever been
found.
• No meiosis occurs.
– Meiosis keeps a check on
homologous chromosome
divergence.
– Rotifer chromosomes no
longer match.
The body cavity
Cavities
(between gut and body wall).
• A body cavity forms in which the organs
can slide about:
Blastocoel
Blastopore
– The PSEUDOCOELOMATE condition is
without a cellular lining.
– With a cellular lining (a peritoneum) it is the
COELOMATE condition.
• This seems to be a prerequisite for
developing a complex body.
A Coelom
A Pseudocoel
• The ‘true’ body cavity
– A body cavity derived from the blastocoel, rather
than from mesoderm
– the coelom is derived from mesoderm
Body covering
(from ectoderm)
Coelom
Pseudocoelomate. Pseudocoelomates
such as nematodes have a body cavity only
partially lined by tissue derived from
mesoderm.
Pseudocoelom
Muscle layer
(from
mesoderm)
Coelomate. Coelomates such as
annelids have a true coelom, a body
cavity completely lined by tissue
derived from mesoderm.
Digestive tract
(from ectoderm)
Body covering
(from ectoderm)
Tissue layer
lining coelom
and suspending
internal organs
(from mesoderm)
Digestive tract
(from endoderm)
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Phylum annelida
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The segmented worms: very successful.
A closed blood system (in vessels).
Varying degrees of cephalisation.
Development (mainly in marine forms) via free living
trochophore larva, but this stage may finish within the egg.
• Some species add segments throughout their lives.
• Skeletal -hydrostatic (using coelom)
• Muscle- longitudinal and circular muscles Each segments muscles
are independent of the other segments.
• Segments bear chaetae (setae).
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