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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 recordHow 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 diversitywithout 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.
Phylum porifera; sponges. assymetrical
Because of assymetry, doesn’t fit into any neat story – so viewed as a development separate from everything else.
Symmetry, bilateral or radial
Jellyfish, corals, radial
cnideria
Radial symmetry plus one ended digestive system = primitive
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 =
Flatworms – one opening dig system, solid (acoelomate)
Round worms; two openings dig system pseudocoel
Seg. Worms (annelids) segments true coelom
Arthropods – trilobite = segmented and legs
Molluscs = chitin, segmented. Odd –legs??
Trilobite = arthropoda
Chitin = mollusca
Marine worm = annelida
All segmented.
Deuterostome, protostome split
Protostomes groups by segmentation.
Link segmentation together
Note: Classical classification((it’s wrong))
New phylogeny
• Lophotrochozoans
• Filter feeders
• Ecdysozoans
• 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.
Central position of flatworms as ancestral.
Question marks.
separate
Ecdysozoans
Shell shedders
segmented
NEXT
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.