113-Intro Animal Diversity-Chapter 32

8/3/2019 113-Intro Animal Diversity-Chapter 32

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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.

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The blind pouchformed by gastru-

lation, calledthe archenteron,

opens to the outsidevia the blastopore.

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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).

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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


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


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


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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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

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113-Intro Animal Diversity-Chapter 32