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Chapter 32
An Introduction to Animal
DiversityEstimates of the number of animalspecies range from 10-20 million to100-200 million.
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What is an Animal (some characteristics of animals)
Multicellular eukaryotic heterotrophs that ingest
their food. Lack the cell walls found in plants and fungi
Have two unique cell types: nerve cells for impulse conductionand muscle cells for movement. Nervous and muscular tissues are unique to animals.
Sexual reproduction (mostly), with diploid stage thatdominates the life cycle
All animals and only animals have Homeobox-containing family of genes (Hoxgenes) whichcontrol development of body form. Produce a wide diversity ofanimal morphology
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Reproduction and Development
Most animals reproduce sexually
With the diploid stage usually dominating thelife cycle
After a sperm fertilizes an egg
The zygote undergoes cleavage, leading to theformation of a blastula
The blastula undergoes gastrulation
Resulting in the formation of embryonic tissuelayers and a gastrula
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Zygote
Cleavage
Eight-cell stage
Cleavage
Blastula Cross sectionof blastula
Blastocoel
Blastocoel
Gastrula Gastrulation
Endoderm
Ectoderm
Blastopore
Early embryonic development in animals
In most animals, cleavage results in theformation of a multicellular stage called a blastula.The blastula of many animals is a hollow ball of cells.
3
The endoderm ofthe archenteron de-
velops into the tissuelining the animals
digestive tract.
6
The blind pouchformed by gastru-
lation, calledthe archenteron,
opens to the outsidevia the blastopore.
5
Most animals also undergo gastrulation, a rearrangement ofthe embryo in which one end of the embryo folds inward, expands,and eventually fills the blastocoel, producing layers of embryonictissues: the ectoderm (outer layer) and the endoderm (inner layer).
4
Only one cleavagestagethe eight-cellembryois shown here.
2The zygote of an animalundergoes a succession of mitoticcell divisions called cleavage.
1
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Reproduction and Development
Some animals develop directly through
transient stages into adults, but others havedistinct larval stages.
Larva is a sexually immature stage that is
morphological distinct from the adult
Larvae undergo metamorphosis, transformingthe animal into an adult
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Evolution of Animals
The history of animals may span more than a billionyears
The animal kingdom includes not only great diversity ofliving species
But the even greater diversity of extinct ones as well
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The common ancestor of living animals
May have lived 1.2 billion800 million years ago
May have resembled modern choanoflagellates,protists that are the closest living relatives ofanimals
Single cell
Stalk
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The common ancestor of living animals
Was probably itself a colonial, flagellatedprotist
Colonial protist,
an aggregate ofidentical cells
Hollow sphere ofunspecialized
cells (shown incross section)
Beginning of cellspecialization
Infolding Gastrula-likeprotoanimal
Somatic cells Digestivecavity
Reproductive cells
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(a) (b)
Neoproterozoic Era (1 Billion524 Million Years Ago)
Early members of the animal fossil record
Include the Ediacaran fauna (From Ediacara Hills of Australia)
Radial bodyform
Segmentedbody form
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Paleozoic Era (542251 Million Years Ago)
The Cambrian explosion
Marks the earliest fossil appearance of manymajor groups of living animals
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Paleozoic Era (542251 Million Years Ago)
What caused the Cambrian explosion
New predator-prey relationships may have generated diversitythrough natural selection.
Predators acquired adaptations to catch prey
Prey acquired adaptations that helped them resist predation
Rise in atmospheric oxygen may have provided opportunities foranimals with higher metabolic rates and larger body sizes
Evolution of the Hoxcomplex provided developmental flexibilitythat resulted in variations in morphology
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Mesozoic Era (25165.5 Million Years Ago)
During the Mesozoic era
Dinosaurs were the dominant terrestrialvertebrates
Coral reefs emerged, becoming important
marine ecological niches for other organisms
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Cenozoic Era (65.5 Million Years Ago to the Present)
The beginning of this era
Followed mass extinctions of both terrestrialand marine animals
Modern mammal orders and insects
Diversified during the Cenozoic
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Animals can be characterized by body plans
One way in which zoologists categorize the diversity ofanimals is according to general features of
morphology and development
A group of animal species that share the same level oforganizational complexity is known as a grade
The set of morphological and developmental traits thatdefine a grade
Are generally integrated into a functional whole
referred to as a body plan
Animals can be categorized according to thesymmetry of their bodies, or lack of it
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Symmetry
Some animals have radial symmetry
Like in a flower pot
Radial symmetry. The parts of aradial animal, such as a sea anemone(phylum Cnidaria), radiate from thecenter. Any imaginary slice throughthe central axis divides the animalinto mirror images.
(a)
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Symmetry
Some animals exhibit bilateral symmetry
Or two-sided symmetry
Bilateral symmetry. A bilateralanimal, such as a lobster (phylum
Arthropoda), has a left side and aright side. Only one imaginary cutdivides the animal into mirror-imagehalves.
(b)
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Symmetry
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 theanimals tissues
Tissues
Are collections of specialized cells isolatedfrom other tissues by membranous layers
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Embryonic tissues
Animal embryos
Form germ layers, embryonic tissues,including ectoderm, endoderm, and mesoderm
Diploblastic animals
Have two germ layers
Triploblastic animals
Have three germ layers
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Body Cavities
In triploblastic animals a body cavity may be
present or absent A true body cavity is called a coelom and is
derived from mesoderm
Coelom
Body covering
(from ectoderm)
Digestive tract(from endoderm)
Tissue layerlining coelomand suspending
internal organs(from mesoderm)
Coelomate. Coelomates such asannelids have a true coelom, a bodycavity completely lined by tissue
derived from mesoderm.
(a)
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Body cavities
A pseudocoelom
Is a body cavity derived from the blastocoel,rather than from mesoderm
PseudocoelomMuscle layer(frommesoderm)
Body covering
(from ectoderm)
Digestive tract(from ectoderm)
Pseudocoelomate. Pseudocoelomatessuch as nematodes have a body cavity onlypartially lined by tissue derived frommesoderm.
(b)
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Body Cavities
Organisms without body cavities
Are considered acoelomates
Body covering
(from ectoderm) Tissue-filled region(frommesoderm)
Digestive tract(from endoderm)
Acoelomate. Acoelomates such asflatworms lack a body cavity betweenthe digestive tract and outer body wall.
(c)
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Protostome and Deuterostome Development Based on certain features seen in early
development
Many animals can be categorized as havingone of two developmental modes: protostomedevelopment or deuterostome development
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Cleavage (spiral versus radial) In protostome development
Cleavage is spiral and determinate (spiral = cell divisiondiagonal to vertical axis of the embryo)
In deuterostome development
Cleavage is radial and indeterminate (radial = thecleavage is either parallel or perpendicular to the vertical axis of the embryo)
Protostome development(examples: molluscs, annelids,
arthropods)
Deuterostome development(examples: echinoderms,
chordates)
Eight-cell stage Eight-cell stage
Spiral and determinate Radial and indeterminate
(a) Cleavage. In general, protostomedevelopment begins with spiral,determinate cleavage.Deuterostome development ischaracterized by radial, indeterminatecleavage.
Cl (d d )
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Cleavage (determinant versus indeterminate) Determinate: Ultimate fate of each cell is
determined (locked-in) early.
Indeterminate: Each early cell retains thepotential to develop into a complete embryo!(this type of cleavage is what makes human identical twins possible)
Protostome development(examples: molluscs, annelids,
arthropods)
Deuterostome development(examples: echinoderms,
chordates)
Eight-cell stage Eight-cell stage
Spiral and determinate Radial and indeterminate
(a) Cleavage. In general, protostomedevelopment begins with spiral,determinate cleavage.
Deuterostome development ischaracterized by radial, indeterminatecleavage.
C l F i
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Coelom Formation
In protostome development
The splitting of the initially solid masses ofmesoderm to form the coelomic cavity is calledschizocoelous development
In deuterostome development
Formation of the body cavity is described asenterocoelous development
Archenteron
Blastopore MesodermCoelom
BlastoporeMesoderm
Schizocoelous: solidmasses of mesodermsplit and form coelom
Enterocoelous:folds of archenteronform coelom
Coelom (b) Coelom formation. Coelomformation begins in the gastrulastage. In protostome development,
the coelom forms from splits in themesoderm (schizocoelousdevelopment). In deuterostomedevelopment, the coelom forms frommesodermal outpocketings of thearchenteron (enterocoelousdevelopment).
F f h Bl
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Fate of the Blastopore
In protostome development
The blastopore becomes the mouth
In deuterostome development
The blastopore becomes the anus
Anus
Anus
Mouth
Mouth
Mouth developsfrom blastopore
Anus developsfrom blastopore
Digestive tube
A i l h l ti t
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Animal phylogenetic tree
Zoologists currently recognize about 35 animalphyla
The current debate in animal systematics
Has led to the development of two
phylogenetic hypotheses, but others exist aswell
A i l h l ti t
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One hypothesis of animal phylogeny basedmainly on morphological and developmentalcomparisons
Animal phylogenetic tree
Porifera
Cnidaria
Ctenophora
Phoronida
Ectoprocta
Brachiopoda
Echinodermata
Chordata
Platyhelminthes
Mollusca
Annelida
Arthropoda
Rotifera
Nemertea
Nematoda
Radiata Deuterostomia Protostomia
Bilateria
Eumetazoa
Metazoa
Ancestral colonialflagellate
A i l h l ti t
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Animal phylogenetic tree
One hypothesis of animal phylogeny basedmainly on molecular data
Calcarea
Silicarea
Ctenophora
Cnidaria
Echinodermata
Chordata
Brachiopoda
Phoronida
Ectoprocta
Platyhelminthes
Nemertea
Mollusca
Annelida
Rotifera
Nematoda
Arthropoda
Radiata
Porifera Deuterostomia Lophotrochozoa Ecdysozoa
Bilateria
Eumetazoa
Metazoa
Ancestral colonial
flagellate
P i t f A t
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Points of Agreement All animals share a common ancestor
Sponges are basal animals
Eumetazoa is a clade of animals with truetissues
Most animal phyla belong to the clade Bilateria
Vertebrates and some other phyla belong to
the clade Deuterostomia
Di t th Bil t i
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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 theprotostomes rather than one: the ecdysozoansand the lophotrochozoans
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Ecdysozoans share a common characteristic
They shed their exoskeletons through aprocess called ecdysis
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Lophotrochozoans
Have a lophophore, a ciliated feeding structure
OR
Go through a distinct larval stage called a
trochophore larva Apical tuftof cilia
Mouth
Anus
(a) An ectoproct, a lophophorate (b) Structure of trochophore larva