The Metazoa

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The Metazoa (Differentiated Multicellular
Animals) An Overview of the Major Groups
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How Do We Classify Life ?
Note Kingdoms Eubacteria and Archaea formerly
classified as Kingdom Monera
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Three Domains
Archaea
Eukarya
Bacteria
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Now Three Domains and 6 Kingdoms
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Conventional Classification of Major Phyla
Protista amoebas, foraminifera,
radiolaria (presumed ancestors of
Metazoa) Metazoa Porifera sponges Cnidaria/Coe
lenterata sea anemones, corals, sea pens,
etc. Several worm phyla flatworms, annelids,
etc. Brachiopoda brachiopods (lamp
shells) Bryozoa bryozoans (moss
animals) Mollusca snails, clams, cephalopods,
etc. Arthropoda shrimps, crabs, insects,
spiders, etc. Echinodermata starfish, sea
urchins, crinoids, etc. Hemichordata acorn
worms, graptolites, etc. Chordata lancelet,
vertebrates, etc.
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Discovery of classic Ediacaran Fauna (about 543
Ma)
Pound Quartzite Ediacara Hills, north of Sydney,
Australia Metazoan Fossils Found in 1946
Classification lumped together as medusoids
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Details originally overlooked A diverse
assemblages of fossils
Mawsonites
Spriggina
Dickinsonia
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Mistaken Point, Newfoundland 565 Ma
spindles
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Portugal Cove Newfoundland Up to about 575 Ma
Charniodiscus (fronds)
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Latest Proterozoic (Ediacaran Period) Oxygenated
atmosphere and seas Complex, soft-bodied metazoa
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Were Vendian organisms Early representatives of
modern phyla ?
Sea Pen
Nudibranch
Ostrich plume hydroid
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Adolph Seilacher Concept of Vendozoa (Extinct
phylum ? Probably not.)
soft bodied quilted structure (fluid-filled
bags ? Dependent on microbial mats mat
stickers fixed to seafloor, photosynthesizers m
at scratchers grazed on microbial mats No
carnivores !
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How do You Make a Metazoan ? You Have to Start
Out Simple.
Single celled Protista
Amoebas, Foraminifera, Radiolaria, etc.
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Phylum Protista the importance of
choanoflagellates
A choanoflagellate is a protist with a collared
cell and a flagellum
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Some choanoflagellates form colonies In such
colonies, all individuals cooperate in moving
their flagella, generating a current from which
food particles can be extracted
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On to the Metazoa
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Phylum Porifera (Sponges) Most Basic Metazoan
Plan of Cowen
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)
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Similar to some of the Ediacaran animals
(remember the frond-like creatures), sponges show
a fractal organization
Leucon-grade sponge (contains multiple sycon
elements)
Sycon-grade sponge (contains multiple ascon
elements)
Ascon-grade sponge
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Phylum Cnidaria / Coelenterata (Second Metazoan
Body Plan of Cowen)
2 layers of tissue ectoderm, endoderm (probably
resulted from invagination of ectoderm)
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Phylum Cnidaria / Coelenterata
hard corals
Hydra
sea pens
sea anemones
soft corals
jellyfish
2 tissue layers ectoderm, endoderm
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Computer-generated fractal
Natural coral
Again, in the more complex forms of these simple
organisms fractal geometry is apparent
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Worms or Bilaterans Most Complex Metazoan
Body Plan of Cowen
triploblastic - 3 principal cell layers
ectoderm, mesoderm, endoderm Basic bilateral
symmetry fractal geometry breaks down, but
tissue differentiation is incredible !
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The Coelom
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)
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Coelom and Orifice Development Protostomes
In the Protostomes (including molluscs, annelid
worms and arthropods), the coelom develops
directly from mesodermal tissue.
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)
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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).
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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. This also meant that
animals could evolve exoskeletons.
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• The Protostomes can be subdivided in two smaller
  groups
• (clades)
• Lophotrochozoa
• Ecdysozoa

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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
Lophophore (in brachiopod)
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Important Lophotrochozoans
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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.
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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
Bivalves
Cephalopods (squids, octopuses, cuttlefish,
ammonoids)
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Lophotrochozoa Phylum Brachiopoda (arm foot)
Key Features pedicle, gut, muscles, lophophore
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Lophotrochozoa Phylum Bryozoa (moss animals)
Key Features colonial habit, lophophore
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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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Important Ecdysozoa
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Ecdysozoa Phylum Arthropoda
Insects Spiders Crabs Lobsters Barnacles Etc.
trilobites
eurypterids
Key Features Jointed appendages 3-fold division
of body (head, thorax, abdomen)
scorpions
shrimps
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Important Deuterostomes (Deuterostomia)
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Deuterostomia Phylum Echinodermata (spiny
skin)
Sea urchins
Brittlestars
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
Acorn worms
Graptolites
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Deuterostomia Phylum Chordata
Key Features notochord dorsal nerve cord,
pharynx gills slits post-anal tail
Sea squirts and salps (Urochordates)
Amphioxus (lancelet) (Cephalochordates)
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END OF LECTURE