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Kingdom Animalia
What is an animal?
- great variety of sizes,
shapes, habitats, etc.
- eukaryotes
- multicellular
- heterotrophs, ingest
food
- no cell walls, held
together by structural
proteins (e.g. collagen)
- many animals have
muscle & nerve cells
Most animals reproduce sexually
- diploid stage usually dominates
Zygote undergoes cleavage: series
of mitotic divisions, without growth
- forms a blastula, often like hollow
ball
Process of gastrulation leads to formation of germ layers &
developmental stage called gastrula
- often a sexually immature larvae, very different from adult
morphology & undergoes metamorphosis into
juvenile, or sexually mature adult
Animals share a set of
genes called Hox genes
that control expression
of many other genes
- determine the pattern
of the body during
development
- set of genes may be
duplicated in some
animals, perhaps allowing
more complex bodies
Animals vary greatly in shape, but often categorized into
major body plans (‘bauplan’), set of morphological &
developmental traits that work together to produce a
successful animal
Animal phyla originally based on distinctive body plans
(though some aspects may evolve multiple times…)
Outdated
Phylogeny
Animals may have no
symmetry (sponges),
or radial symmetry:
plane in any direction
creates mirror image
- have dorsal (top) &
ventral (bottom)
sides
- or bilateral symmetry:
only one plane for
mirror images
- have left/right sides
& anterior (front)
& posterior (back)
as well
Animals with bilateral symmetry typically show
cephalization: concentration of sense organs &
neural tissue at anterior end
Most animals have tissues:
collections of specialized
cells, separated by
membranes
Tissues develop from germ layers formed during
gastrulation
Ectoderm – outer surface
- forms outer covering of animal, & sometimes central
nervous system
Endoderm – inner, lines archenteron
(space in developing digestive tube)
- forms digestive tract, liver &
lungs of vertebrates
If only these 2 layers,
= diploblastic
(cnidarians &
comb jellies)
Bilateral animals develop
3 germ layers (thus
triploblastic), with
mesoderm between
ectoderm & endoderm
- forms muscles &
other internal organs
Most triploblastic
animals develop a
body cavity known
as a coelom
- fluid or air-filled space
between digestive cavity
& body covering
Coelomates: ‘true’ coelom
bounded by mesoderm on
inner & outer surfaces
- surrounding tissue also
suspends internal organs
Pseudocoelomates: no
mesoderm separating
cavity from digestive
tract
Acoelomates: no body
cavity (mesoderm fills
space between ectoderm
& endoderm)
Coelom serves several functions:
- cushions & protects organs
- hydrostatic skeleton
- allows organs to grow & move
independently
Coeloms & pseudocoeloms appear to
have evolved several times, not
associated with monophyletic groups
Animals also
categorized by
patterns of
development as
Protostomes or
Deuterostomes
Protostomes have spiral cleavage (cells not aligned vertically)
& each cell has fate determined early
Deuterostomes have radial cleavage & cells remain capable
of forming any organ until much later
Also differ in how coelom forms: in protostome cavity
develops in center of mesoderm mass
- in deuterostomes coelom ‘buds off’ from archenteron
cavity
Also differ in what forms from blastopore: mouth in
protostomes (protos – first; stoma – mouth)
- anus in deuterostomes (deuteros – second), mouth
forms later at opposite end
Who are the closest relatives of animals?
Not so distant from fungi in tree of life, but not closest…
Within Unikonta, closest kin are
indicated as choanoflagellates
Choanoflagellates morphologically very similar to some
cells in sponges (& some other animals…)
- DNA sequences & shared proteins indicate relationship
- choanoflagellates are filter-feeders in salt & fresh water,
solitary or colonial
Animalia (or Metazoa) is agreed to be monophyletic,
originating 675-875 MYA
- beyond that, there has
been debate, but a
clearer picture is
emerging
The old ‘morphological/
developmental’ phylogeny
should probably be
disregarded in favor of the
changes in the molecular
phylogeny
Phylum Porifera – Sponges
- earliest branch in phylogeny
- recent studies indicate
they are monophyletic,
though several ancient
& distinct groups
within the taxon
Sponges are multicellular, but lack true tissues
- sessile filter feeders, from a few mm to a few metres
- water drawn in through pore into central spongocoel,
out through osculum
No tissues, but several cell types
- choanocytes line spongocoel, flagellae create currents
- outer epidermis with doughnut-shaped cells around pores
- inner & outer layers separated by gelatinous mesohyl
- amoebocytes
move through
mesohyl,
distribute
food & make
spicules
(silica or
calcium
carbonate)
- most are sequential hermaphrodites (either produce eggs
first or sperm, then switch)
- about 5,500 spp., mostly marine but 14 spp. in IL
Remaining animals in Eumetozoa, have true tissues
First 2 major branches
both have radial
symmetry & are
diploblastic (no
mesoderm, but
‘jelly’ between
ectoderm &
endoderm)
Phylum Ctenophora – Comb jellies
Small phylum (100+ spp.), but can be abundant in ocean
- resemble jellyfish somewhat, propelled by 8 ‘combs’
of cilia
- capture prey with
sticky tentacles
Phylum Cnidaria – Cnidarians (sea anemones, jellyfish, etc.)
Jellyfish Lake, Palau
Diverse phylum (10,000 spp.) divided into 4 classes,
but share simple radial, diploblastic body plan
- 2 forms, typically sessile polyp & mobile medusa
- central gastrovascular cavity with single opening (mouth
& anus)
Tentacles around mouth for capturing prey, covered with
cnidocytes
- contain cnidae, ‘explosive’ organelles
- many contain
nematocysts –
stinging thread
with venom
can penetrate
prey & hold it
Have contractile tissues & a nerve net (but no brain)
to coordinate movement, sensory structures around
body
Class Hydrozoa – hydras, Portuguese man-of-wars, etc.
- usually alternate between medusa & polyp stages
- freshwater hydras only as polyps
- polyps typically asexual, sexual reproduction by medusae
Class Scyphozoa – jellyfish (‘jellies’), sea nettles
- only ~200 spp., all marine
- coastal spp. may have polyps, but others lack them
Class Cubozoa – Box jellies, sea wasps
- only ~30 spp. all marine, usually tropical
- ‘cube’ or ‘box’ shaped medusa, complex eyes on fringe of
medusae
- may be dangerously toxic
Class Anthozoa – Sea anemones & corals
- largest class (~6,000 spp.), all marine
- only polyp stage
Corals may be solitary or colonial
- may secrete calcium carbonate skeleton
- coral reefs are extremely productive & diverse marine
habitats
Remaining phyla all belong to
the Bilateria
- bilateral symmetry
- three germ layers
Phylum Acoela – Acoelomate flatworms
- small group (~400 spp.), tiny marine animals, formerly
included in Platyhelminthes
- branched off before 3 main groups of bilateria, may
suggest ancestral traits
- very simple anatomy: no
coelom, no true gut (no
epithelial lining), nerves
slightly denser at front end
Next section of phylogeny
includes mostly animals
with ‘protostome’ style of
development
Divided into 2 main groups,
Lophotrochozoa & Ecdysozoa
Lophotrochozoa - 18 phyla with diverse body plans
recognized by genetic similarity
- some use a lophophore w. ciliated tentacles for feeding
- others have a trochophore larval stage
- but…
others have
neither
First phylum in Lophotrochozoa is Platyhelminthes – the
Flatworms - ~20,000 spp.?
- also acoeolomate & triploblastic, but usually more complex
than Acoela
- gastrovascular cavity does have epithelium, distinct ganglia
at anterior end of 2 ventral nerve cords
- thin dorsoventrally flattened body, often w. cilia
- no respiratory organs or circulatory system
- simple excretory system mostly to maintain osmotic
balance
Divided into 4 classes: Class Turbellaria
- most free-living (unlike other classes)
- most marine (some large & beautiful), a few freshwater
(e.g. Dugesia, planarians)
- glide w. ventral cilia & mucus, or swim w. undulating
muscles
- some reproduce by fission, or sexual reproduction by crossfertilizing hermaphrodites
Class Monogenea – monogeneans
- external parasites of fish (or hippo) with relatively simple
life cycles