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IB-202-1 (3-8-06)
An Introduction to
Animal Diversity
Chapter 32
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Much Diversity in the Marine Environment (Oceans)
• Overview: Welcome to Your Kingdom
• The animal kingdom
– Extends far beyond humans and other animals
we may encounter
Figure 32.1
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• Concept 32.1: Animal are multicellular,
heterotrophic eukaryotes with tissues that
develop from embryonic layers
• Several characteristics of animals
– Sufficiently define the group
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Nutritional Mode
• Animals are heterotrophs
– That ingest their food (reduced carbon)
– They lack cell walls (no cellulose)
– Their bodies are held together by structural
proteins such as collagen
•
Nervous tissue and muscle tissue are
unique to animals
– Most animals reproduce sexually (egg and
sperm with each contributing a haploid
number of chromosomes to the zygote)
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Cell Structure and Specialization
• Animals are multicellular eukaryotes
• Animal cells lack cellulose cell walls which
means that they cannot tolerate intracellular
pressure like plant cells. If they swell they will
burst.
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Reproduction and Development
• Most animals reproduce sexually
– With the diploid stage usually dominating the
life cycle
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• After a sperm fertilizes an egg
– The zygote undergoes cleavage, leading to the
formation of a blastula
• The blastula undergoes gastrulation
– Resulting in the formation of embryonic tissue
layers and a gastrula
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• Early embryonic development in animals
1 The zygote of an animal
undergoes a succession of mitotic
cell divisions called cleavage.
2 Only one cleavage
stage–the eight-cell
embryo–is shown here.
3 In most animals, cleavage results in the
formation of a multicellular stage called a blastula.
The blastula of many animals is a hollow ball of cells.
Blastocoel
Cleavage
Cleavage
6 The endoderm of
the archenteron develops into the tissue
lining the animal’s
digestive tract.
Zygote
Eight-cell stage
Blastula
Cross section
of blastula
Blastocoel
Endoderm
5 The blind pouch
formed by gastrulation, called
the archenteron,
opens to the outside
via the blastopore.
Ectoderm
Gastrula
Gastrulation
Blastopore
Figure 32.2
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4 Most animals also undergo gastrulation, a rearrangement of
the embryo in which one end of the embryo folds inward, expands,
and eventually fills the blastocoel, producing layers of embryonic
tissues: the ectoderm (outer layer) and the endoderm (inner layer).
• All animals, and only animals
– Have Hox genes that regulate the
development of body form
• Although the Hox family of genes has been
highly conserved
– It can produce a wide diversity of animal
morphology
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• The common ancestor of living animals
– May have lived 1.2 billion–800 million years ago
– May have resembled modern choanoflagellates,
protists that are the closest living relatives of
animals
Single cell
Stalk
Figure 32.3
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– Was probably itself a colonial, flagellated
protist
Digestive
cavity
Somatic cells
Reproductive cells
Colonial protist,
an aggregate of
identical cells
Hollow sphere of
unspecialized
cells (shown in
cross section)
Beginning of cell
specialization
Figure 32.4
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Infolding
Gastrula-like
“protoanimal”
Paleozoic Era (542–251 Million Years Ago)
• The Cambrian explosion
– Marks the earliest fossil appearance of many
major groups of living animals
– Is described by several current hypotheses
Figure 32.6
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• Animals can be characterized by “body plans”
• One way in which zoologists categorize the
diversity of animals
– Is according to general features of morphology
and development
• A group of animal species
– That share the same level of organizational
complexity is known as a grade
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• The set of morphological and developmental
traits that define a grade
– Are generally integrated into a functional whole
referred to as a body plan
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Symmetry
• Animals can be categorized
– According to the symmetry of their bodies, or
lack of it
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• Some animals have radial symmetry
– Like in a flower pot
(a) Radial symmetry. The parts of a
radial animal, such as a sea anemone
(phylum Cnidaria), radiate from the
center. Any imaginary slice through
the central axis divides the animal
into mirror images.
Figure 32.7a
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• Some animals exhibit bilateral symmetry
– Or two-sided symmetry
(b) Bilateral symmetry. A bilateral
animal, such as a lobster (phylum
Arthropoda), has a left side and a
right side. Only one imaginary cut
divides the animal into mirror-image
halves.
Figure 32.7b
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• Bilaterally symmetrical animals have
– A dorsal (top) side and a ventral (bottom) side
– A right and left side
– Anterior (head) and posterior (tail) ends
– Cephalization, the development of a head
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Tissues
• Animal body plans
– Also vary according to the organization of the
animal’s tissues
• Tissues
– Are collections of specialized cells isolated
from other tissues by membranous layers
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• Animal embryos
– Form germ layers, embryonic tissues,
including ectoderm, endoderm, and mesoderm
• Diploblastic animals
– Have two germ layers (ectoderm & endoderm)
• Triploblastic animals
– Have three germ layers (ectoderm, endoderm
and mesoderm. Mesodermal layer is derived
from endoderm cells)
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Body Cavities
• In triploblastic animals
– A body cavity may be present or absent
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• A true body cavity
– Is called a coelom and is derived from
mesoderm
Coelom
(a) Coelomate. Coelomates such as
annelids have a true coelom, a body
cavity completely lined by tissue
derived from mesoderm.
Tissue layer
lining coelom
and suspending
internal organs
(from mesoderm)
Digestive tract
(from endoderm)
Figure 32.8a
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Body covering
(from ectoderm)
• A pseudocoelom
– Is a body cavity derived from the blastocoel,
rather than from mesoderm
Body covering
(from ectoderm)
(b) Pseudocoelomate. Pseudocoelomates
such as nematodes have a body cavity only
partially lined by tissue derived from
mesoderm.
Pseudocoelom
Digestive tract
(from ectoderm)
Figure 32.8b
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Muscle layer
(from
mesoderm)
• Organisms without body cavities
– Are considered acoelomates
Body covering
(from ectoderm)
(c) Acoelomate. Acoelomates such as
flatworms lack a body cavity between
the digestive tract and outer body wall.
Digestive tract
(from endoderm)
Figure 32.8c
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Tissuefilled region
(from
mesoderm)
Developmental Patterns (Deuterostome and
Protostome)
• Based on certain features seen in early
development
– Many animals can be categorized as having
one of two developmental modes: protostome
development or deuterostome development
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Cleavage
• In protostome development
– Cleavage is spiral and determinate
• In deuterostome development
– Cleavage is radial and indeterminate
Protostome development
(examples: molluscs, annelids,
arthropods)
Eight-cell stage
Spiral and determinate
Deuterostome development
(examples: echinoderms,
chordates)
Eight-cell stage
Radial and indeterminate
Figure 32.9a
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(a) Cleavage. In general, protostome
development begins with spiral,
determinate cleavage.
Deuterostome development is
characterized by radial, indeterminate
cleavage.
Coelom Formation
• In protostome development
– The splitting of the initially solid masses of
mesoderm to form the coelomic cavity is called
schizocoelous development
• In deuterostome development
– Formation of the body cavity is described as
enterocoelous development
Coelom
Archenteron
Coelom
Mesoderm
Blastopore
Mesoderm
Blastopore
Enterocoelous:
Schizocoelous: solid
folds of archenteron
masses of mesoderm
form coelom
split and form coelom
Figure 32.9b
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(b) Coelom formation. Coelom
formation begins in the gastrula
stage. In protostome development,
the coelom forms from splits in the
mesoderm (schizocoelous
development). In deuterostome
development, the coelom forms from
mesodermal outpocketings of the
archenteron (enterocoelous
development).
Fate of the Blastopore
• In protostome development
– The blastopore becomes the mouth
• In deuterostome development
– The blastopore becomes the anus
Mouth
Anus
Digestive tube
Mouth
Figure 32.9c
Mouth develops
from blastopore
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Anus
Anus develops
from blastopore
• Concept 32.4: Leading hypotheses agree on
major features of the animal phylogenetic tree
• Zoologists currently recognize about 35 animal
phyla
• The current debate in animal systematics
– Has led to the development of two
phylogenetic hypotheses, but others exist as
well
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“Radiata”
Deuterostomia
Metazoa
Figure 32.10
Ancestral colonial
flagellate
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Nematoda
Nemertea
Rotifera
Arthropoda
Annelida
Protostomia
Bilateria
Eumetazoa
Mollusca
Platyhelminthes
Chordata
Echinodermata
Brachiopoda
Ectoprocta
Phoronida
Ctenophora
Cnidaria
Porifera
• One hypothesis of animal phylogeny based
mainly on morphological and developmental
comparisons
Arthropoda
Nematoda
Rotifera
Annelida
Mollusca
Nemertea
Platyhelminthes
Ectoprocta
Phoronida
Brachiopoda
Chordata
Echinodermata
Cnidaria
Ctenophora
Silicarea
Calcarea
• One hypothesis of animal phylogeny based
mainly on molecular data
“Radiata”
“Porifera”
Deuterostomia
Lophotrochozoa
Bilateria
Eumetazoa
Metazoa
Figure 32.11
Ancestral colonial
flagellate
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Ecdysozoa
Points of Agreement
• All animals share a common ancestor
• Sponges are basal animals
• Eumetazoa is a clade of animals with true
tissues
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• Most animal phyla belong to the clade Bilateria
• Vertebrates and some other phyla belong to
the clade Deuterostomia
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Disagreement over the Bilaterians
• The morphology-based tree
– Divides the bilaterians into two clades:
deuterostomes and protostomes
• In contrast, several recent molecular studies
– Generally assign two sister taxa to the
protostomes rather than one: the ecdysozoans
and the lophotrochozoans
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• Ecdysozoans share a common characteristic
– They shed their exoskeletons through a
process called ecdysis
Figure 32.12
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• Lophotrochozoans share a common characteristic
– Called the lophophore, a feeding structure
• Other phyla
– Go through a distinct larval stage called a
trochophore larva
Apical tuft
of cilia
Mouth
Figure 32.13a, b
(a) An ectoproct, a lophophorate
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Anus
(b) Structure of trochophore larva
Future Directions in Animal Systematics
• Phylogenetic studies based on larger
databases
– Will likely provide further insights into animal
evolutionary history
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