Animal diversity and
relationships
Living forms
• At least 30 phyla
• But only x “important”
ones
Importance =
numerous,
ecologically
important, and fit into
our conceptions of
evolution
Porifera = sponges
Assymetrical, 2 cell
layers , internal silica
spicules = skeleton.
Cnideria, jellyfish, corals
Radially symmetrical, 2 cell
layers, one ended digestive
system. Often with algae
inside to provide nourishment
Light sensitivity.
Ctenophora – comb jellies
Bilateral, one ended gut, marine
Platyhelminthes = flat worms. Three cell
layers, no body cavity, one ended digestive
system, brain, nerves, many are parasitic.
Two ended digestive system
Head and tail, have an internal
cavity – not solid.
Very numerous in soil.
Annelida = segmented worms; bigger most free living. Have nerves, three
cell layers, true body cavity = coelom, circulatory system, excretory
system. Skin breathing and many have legs. Some carnivorous, soil eaters
or leaches – blood suckers
Mollusca: clams, snails, squid, octopus, slugs
Two ended gut, nerves, circulation, filter
feeders or carnivorous. Some (squid, octopus)
with eyes, brain, etc.
Remnants of segmentation – chitins.
Arthropods – joint leg = insects, crustacea
Segmented, brain, sense organs, excretory
system
Echinoderms – pentaradiate,
Starfish, urchins, sea lillys.
Chordata, including vertebrata . Backboned
organisms.
The fossil record
How to determine relationships – try fossils first.
Burgess Shale in British Columbia, a unique preservation at the right age.
Segmented worms (with legs?)
arthropods
Chordate – similar to early
members of this group
sponge
????
Result of fossil record: all major groups plus some other odd ones appear
almost simultaneously = no sequental appearance.
How to make sense out of diversity
without fossils.
Rules: simple to complex
symmetry (assymetrical, radial, bilateral)
Cell layers – 1 to 2 to 3
Embryology - (old ontogeny recapitulates
phylogeny statement of Haeckel)
Digestive system design; one ended to two
ended
Ernst Haeckel, 1866;
“ontogeny recapitulates
phylogeny” = early stages
of development mirror
evolutionary changes.
Note – gill clefts in all
embryos, tail present, etc.
Basis; development is
often incremental – add
new stages to old –
harder to change the
beginning stages.
Classic example of this, the
aortic arches in vertebrates.
Start with a complete set;
need gill arches to deposit
them.
All embryos have gill clefts
and a complete set of arches
To get to adult stages, loose
some of the arches.
Because of assymetry, doesn’t fit into
any neat story – so viewed as a
development separate from everything
else.
Phylum porifera;
sponges.
assymetrical
Symmetry, bilateral or radial
Radial symmetry plus one
ended digestive system =
primitive
Jellyfish, corals, radial
cnideria
Bilateral symmetry; all other groups.
Clues from development
1. Hollow ball of cells
2. Then 2 cell layers, one
opening,radial symmetry= cnidaria
3. Then three cell layers – new opening
bilateral symmetry = all higher forms
Cell layers 2 vs 3. sponges and cnideria = 2
Digestive system – one ended vs. two ended
Flatworms – bilaterial but with one openning to
digestive system
Other ‘worms’ – two ended digestive system.
So: 1 – no symmetry, followed by radial and then bilateral - porifera oldest
then cnidaria, then everything else
2. one ended gut, followed by 2 ended.
3. 2 cell layers, followed by three
Now what?
Use embryology – the great Protostome – Deuterostome split
Protostomes vs deuterostomes
•
•
•
•
•
First opening = mouth
Determinate clevage
Spiral clevage
Mesoderm = 4d cell
Schizocoel coelom
•
•
•
•
•
First opening = anus
Indeterminate clevage
Radial clevage
Mesoderm = infolding
Endocoel coelom
Protostomes – how to organize
• Classical method
mesoderm – solid to pseudocoelom to true coelom
one ended gut to two ended
no segments to segments
Body cavity (coelom) in relation to mesoderm
Solid mesoderm = no coelom = flatworms
acoelomate
Coelom partially lined with mesoderm =
roundworms
pseudocoele
Coelom completely lines with mesoderm =
Segmented worms, vertebrates, etc.
true coelom
Result =
Molluscs = chitin, segmented. Odd –legs??
Arthropods – trilobite = segmented and
legs
Seg. Worms (annelids) segments true coelom
Round worms; two openings dig system pseudocoel
Flatworms – one opening dig system, solid (acoelomate)
Trilobite = arthropoda
Chitin = mollusca
Marine worm = annelida
All segmented.
Deuterostome, protostome split
Note: Classical classification
Protostomes groups by
segmentation.
Link segmentation together
((it’s wrong))
New phylogeny
• Lophotrochozoans
• Ecdysozoans
• Filter feeders
• Shell shedders
Two phylogenies: left based on hypothesized relationships, right based on
Both genetic similarity and time. Right is correct in that all major groups
appear almost simultaneously (brushpile evolution)
Note: flatworms,mollusc and annelid together// no arthropods
Arthropods and round worms in this group.
N
E
X
T
Ecdysozoans
Shell shedders
separate
Central position of flatworms as ancestral.
Question marks.
segmented
How come the old phylogeny wrong?
1. Segmentation arose more than once – not a unifying trait
2. Some organisms, especially parasites (flatworms, roundworms) may have
gotten simpler in structure through time.
3. A poor choice of “unifying” characters.