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The Metazoa
(Differentiated Multicellular Animals)
An Overview of the Major Groups
How Do We Classify Life ?
Note: Kingdoms Eubacteria and Archaea formerly
classified as Kingdom Monera
Three Domains
Now Three Domains and 6 Kingdoms
Archaea
Eukarya
Bacteria
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Conventional Classification of Major Phyla
Protista
amoebas, foraminifera, radiolaria
(presumed ancestors of Metazoa)
Discovery of classic “Ediacaran Fauna”
(about 543 Ma)
Metazoa
Porifera
Cnidaria/Coelenterata
Several “worm” phyla
Brachiopoda
Bryozoa
Mollusca
Arthropoda
Echinodermata
Hemichordata
Chordata
sponges
sea anemones, corals, sea pens, etc.
flatworms, annelids, etc.
brachiopods (“lamp shells”)
bryozoans (“moss animals”)
snails, clams, cephalopods, etc.
shrimps, crabs, insects, spiders, etc.
starfish, sea urchins, crinoids, etc.
acorn worms, graptolites, etc.
lancelet, vertebrates, etc.
Pound Quartzite
Ediacara Hills, north of Sydney, Australia
Metazoan Fossils Found in 1946
Classification: lumped together as “medusoids”
Details originally overlooked:
A diverse assemblages of fossils
Mistaken Point, Newfoundland
565 Ma
Mawsonites
Spriggina
Dickinsonia
“spindles”
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Portugal Cove Newfoundland
Up to about 575 Ma
Charniodiscus (fronds)
Were Vendian organisms
Early representatives of modern phyla ?
Sea Pen
Latest Proterozoic (Ediacaran Period)
Oxygenated atmosphere and seas
Complex, soft-bodied metazoa
Adolph Seilacher
Concept of “Vendozoa:
(Extinct phylum ? Probably not.)
Nudibranch
soft bodied
“quilted” structure (fluid-filled bags ?”
Dependent on microbial mats
“mat stickers”: fixed to seafloor, photosynthesizers
“mat scratchers”: grazed on microbial mats
Ostrich plume hydroid
No carnivores !
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How do You Make a Metazoan ?
You Have to Start Out Simple.
Phylum Protista: the importance of choanoflagellates
Single celled Protista
A choanoflagellate is a protist with a
collared cell and a flagellum
Amoebas, Foraminifera, Radiolaria, etc.
Some choanoflagellates form colonies
In such colonies, all individuals cooperate in moving
their flagella, generating a current from which food
particles can be extracted
On to the Metazoa…
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Phylum Porifera (Sponges)
Most Basic Metazoan Plan of Cowen
Similar to some of the Ediacaran
animals (remember the frond-like
creatures), sponges show a fractal
organization
Leucon-grade sponge
(contains multiple
“sycon” elements)
Single layer of tissue
(collared cells)
Sponges also have collared cells, but these form a larger, integrated
structure supported by rigid spicules or organic tissue. The differentiation
of cells required the evolution of Hox Genes (genes that dictate differing
functions of cells)
Sycon-grade sponge
(contains multiple
“ascon” elements)
Ascon-grade sponge
Phylum Cnidaria / Coelenterata
Phylum Cnidaria / Coelenterata
(Second Metazoan Body Plan of Cowen)
Hydra
2 layers of tissue:
ectoderm, endoderm
(probably resulted from invagination of ectoderm)
hard corals
soft corals
jellyfish
sea pens
sea anemones
2 tissue layers: ectoderm, endoderm
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“Worms” or “Bilaterans”
Most Complex Metazoan Body Plan of Cowen
Natural coral
Computer-generated
fractal
Again, in the more complex forms of these simple organisms
fractal geometry is apparent
The Coelom
triploblastic - 3 principal cell layers
ectoderm, mesoderm, endoderm
Basic bilateral symmetry: fractal geometry breaks down,
but tissue differentiation is incredible !
Coelom and Orifice Development;
Protostomes
In the Protostomes (including
molluscs, annelid worms and
arthropods), the coelom
develops directly from
mesodermal tissue.
The Ectoderm and Endoderm can be viewed as essentially
solid, continuous layers.
The Mesoderm is a little more complicated in that it actually
lines a fluid-filled body cavity called the coelom. It is within
the coelom that internal organs other than the gut develop
(e.g. respiratory organs)
Another distinguishing
characteristic to the
protostomes is the
development of the mouth
before the anus in the
young embryo
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Coelom and Orifice Development;
Deuterostomes
In the Deuterostomes
(including echinoderms and
chordates), the coelom
develops from outpockets of
the gut (endoderm)
Another distinguishing
characteristic to the
protostomes is the
development of the
anus before the mouth in the
young embryo (blastophore)
The Evolution of the Coelom
The coelom may have initially evolved as a hydraulic device.
A bilateran with a coelom can squeeze its internal fluids with
body muscles.
This squeezing bulges the body wall at the weakest point,
and can be used as a “power drill” for burrowing (think about
how a worm gets around).
The Evolution of the Coelom
In addition, this pumping could facilitate the transport of
oxygen through the body without relying on the bathing of
tissues in oxygenated water by diffusion through a thin
ectoderm.
This means that animals could efficiently deliver oxygen
throughout their bodies without compromising the
effectiveness of their outer skins (ectoderm) or size.
The Protostomes can be subdivided in two smaller groups
(clades):
1. Lophotrochozoa
2. Ecdysozoa
This also meant that animals could evolve exoskeletons.
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Lophotrochozoa: This group gets
its confusing name from two
related subgroups (linked by
molecular phylogenetic studies):
1. The trochozoa - animals with
distinctive, fuzzy, trochophore
larvae, which include the phyla
Platyhelminthes and the Mollusca.
2. The lophophora – animals which
feed via a fringe of hollow
tentacles, called a lophophore),
which include the phyla
Brachiopoda and Bryozoa.
trochophore
larva
Important Lophotrochozoans
Lophophore
(in brachiopod)
Lophotrochozoa: Phylum Platyhelminthes (flatworms)
Flatworms do not have a coelom, and it is likely that
something like a flatworm gave rise to more advanced
coelomate bilaterans.
Lophotrochozoa: Phylum Mollusca
Each class derived from HAM
(hypothetical ancestral mollusc)
Key Features:
gut
mantle cavity
radula (rasping organ)
gills
foot
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Lophotrochozoa: Phylum Mollusca
Gastropods
Lophotrochozoa: Phylum Brachiopoda (“arm foot”)
Bivalves
Key Features: pedicle, gut, muscles, lophophore
Cephalopods (squids, octopuses, cuttlefish, ammonoids)
Lophotrochozoa: Phylum Bryozoa (“moss animals”)
Key Features:
colonial habit,
lophophore
Ecdysozoa: This
group includes
animals that moult
their outer covering
as they grow.
Phylum Arthropoda is
the primary phylum of
this group.
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Ecdysozoa: Phylum Arthropoda
Important Ecdysozoa
trilobites
eurypterids
Insects
Spiders
Crabs
Lobsters
Barnacles
Etc.
Key Features:
Jointed appendages
3-fold division of body
(head, thorax, abdomen)
scorpions
shrimps
Deuterostomia: Phylum Echinodermata (“spiny skin”)
Sea urchins
Brittlestars
Important Deuterostomes (Deuterostomia)
Crinoids
Starfish
Sea cucumbers
Key Features: 5-fold symmetry, calcite plates
(but embryos are bilateral, suggesting a
bilateral ancestor)
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Deuterostomia: Phylum Hemichordata
Key Characteristics:
3-part division of body
(preoral lobe, collar, trunk)
Pharynx
Gill slits
Stomochord
Pterobranchs
Graptolites
Acorn worms
Deuterostomia: Phylum Chordata
Sea squirts and salps (Urochordates)
Key Features:
notochord
dorsal nerve cord,
pharynx
gills slits
post-anal tail
Amphioxus (lancelet)
(Cephalochordates)
END OF LECTURE
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