chapter 19 evolution of the animal phyla. general features of animals animals share many important...

CHAPTER 19

EVOLUTION OF THE ANIMAL PHYLA

GENERAL FEATURES OF ANIMALS

• Animals share many important characteristics, such as they:• are heterotrophs• are multicellular and lack cell walls• can move from place to place• have diverse forms and habitats• reproduce, mostly by sexual reproduction• have a common pattern of development • have unique tissues

THE ANIMAL FAMILY TREE

• Animals consist of 35 very different phyla.• To judge which phyla are more closely related,

taxonomists traditionally have compared anatomical features and aspects of embryological development.

• The end result are phylogenies, which are basically like family trees.


• The kingdom Animalia is traditionally divided into two main branches based on tissue presence.• Parazoa possess neither tissues nor organs

and have no discernible symmetry.• They are represented mostly by the phylum

Porifera, the sponges.• Eumetazoa have a definite shape and

symmetry and, in most cases, tissues organized into organs and organ systems.

• Although very different, the Parazoa and Eumetazoa are thought to have evolved from a common ancestor.


• Eumetazoans are divided into two groups:• Radiata have radial symmetry and two

embryological layers, an outer ectoderm and an inner endoderm.• This body plan is called diploblastic.

• Bilateria have bilateral symmetry and a third embryological layer, the mesoderm, that occurs between the ectoderm and the endoderm.• This body plan is called triploblastic.


• Traditionally, the Bilateria have been subdivided based on traits that were important to the evolutionary history of phyla.• For example, the presence or absence of a body

cavity.

• The traditional animal phylogeny relies on the either-or nature of categories.

THE ANIMAL FAMILY TREE: THE TRADITIONAL VIEWPOINT

Notissues

Sponges Cnidarians Ctenophorans Flatworms Nematodes Mollusks Annelids Arthropods Lophophorates Echinoderms Chordates

Deuterostomes

Coelom forms fromembryonic gut

Mix ofcharacteristics

Coelom forms frommesoderm cell mass

Parazoa

Radialsymmetry

No coelom

Acoelomates

Pseudocoel

Pseudo-coelomates

Coelomates

Mix ofcharacteristics

ProtostomesRadiata


• The traditional animal phylogeny is being revised because some of the important characters may not be conserved to the extent previously thought.• Molecular systematics offers a means to

construct phylogenic trees using unique gene sequences as means to detect relatedness.

• New molecular data has resulted in a variety of new phylogenies, most of which divide protostomes into:• Lophotrochozoans• Ecdysozoans

THE ANIMAL FAMILY TREE: A NEW LOOK

Grow by increasing body mass, locomotion is ciliary, trochophore larvae

Grow by molting Coelom forms fromembryonic gut

Radialsymmetry

Notissues

Sponges Cnidarians Ctenophorans Lophophorates Flatworms Mollusks Annelids Nematodes Arthropods Echinoderms Chordates

Parazoa Radiata Protostomes Deuterostomes

EcdysozoaLophotrochozoa

SIX KEY TRANSITIONS IN BODY PLAN

• The evolution of animals is marked by six key transitions in body plan:

1. tissues2. bilateral symmetry3. body cavity4. segmentation 5. molting6. deuterostome development


• The presence of tissues is the first key transition in the animal body plan.• Only the Parazoa, the sponges, lack defined

tissues and organs.• These animals exist as aggregates of cells

with minimal intercellular coordination.• All other animals, the Eumetazoa, possess

tissues.


• Virtually all animals other than sponges have a definite shape and symmetry.• Radial symmetry is a body plan in which all

parts of the body are arranged around a central axis.• Any plane passing through the central axis

divides the organism in halves that are approximate mirror images.

• Bilateral symmetry is a body plan with distinct right and left halves that are mirror images.• The plan allows for specialization among body

regions and more efficient movement.


• The evolution of a body cavity was an important step in animal evolution.• This internal space allowed for the support of

organs, distribution of materials, and coordination of development.

• For example, the digestive tract can be larger and longer.


• Bilateral animals can be divided into two groups based on differences in the basic pattern of development.• Protostomes include the flatworms,

nematodes, mollusks, annelids, and arthropods.• Deuterostomes include the echinoderms and

the chordates.• Deuterostomes evolved from protostomes more

than 630 million years ago.


• The subdivision of the body into segments is another key transition in the animal body plan.• In highly segmented animals, each segment

can develop a more or less complete set of adult organ systems.

• Each segment can function as a separate locomotory unit.

EVOLUTIONARY TRENDS AMONG THE ANIMALS Deuterostome development,

endoskeleton

?

Parazoa

Segmented

Deuterostomes

Coelomates

Eumetazoa

BilateriaRadiata

Acoelomates

Protostomes

Pseudocoelomates

Segmented

Notochord,segmentation,jointedappendages

Coelom

Body cavity

Bilateral symmetry

Tissues

Multicellularity

Ancestral protist

No true tissues

Radial symmetry

Pseudocoel

Po

rife

ra (

sp

on

ges

)

Ra

dia

ta (

Cn

idar

ia a

nd

Cte

no

ph

ora

)

Pla

tyh

elm

inth

es

(fla

two

rms

)

Ne

ma

tod

a (

rou

nd

wo

rms

)

Ro

tife

ra (

roti

fers

)

Mo

llu

sca

Molting

Segmentation

An

nel

ida

Art

hro

po

da

Lo

ph

op

ho

rate

s

Jointed appendagesexoskeleton, molting

Ch

ord

ata

Ech

ino

de

rmat

a

Most successful of all animal phyla; chitinous exoskeletoncovering segmented bodies with paired, jointed appendages;most insect groups have wings; nearly all are freshwater orterrestrial

Soft-bodied coelomates whose bodies are divided into threeparts: head-foot, visceral mass, and mantle; many have shells;almost all possess a unique rasping tongue called a radula; mostare marine or freshwater but 35,000 species are terrestrial

Mammals, fish,reptiles, birds,amphibians

Segmented coelomates with a notochord; possess a dorsal nervecord, pharyngeal pouches, and a tail at some stage of life; invertebrates, the notochord is replaced during development by thespinal column; most are marine, many are freshwater, and 20,000species are terrestrial

Platyhelminthes(flatworms)

Planaria,tapeworms,flukes

Segmented coelomates with a notochord; possess a dorsal nervecord, pharyngeal pouches, and a tail at some stage of life; invertebrates, the notochord is replaced during development by thespinal column; most are marine, many are freshwater, and 20,000species are terrestrial

Ascaris, pinworms,hookworms, Filaria

Earthworms,marine worms,leeches

Coelomate, serially segmented, bilaterally symmetrical worms;complete digestive tract; most have bristles called setae on eachsegment that anchor them during crawling; marine, freshwater,and terrestrial

Phylum

ApproximateNumber ofNamed Species

1,000,000

Key Characteristics

110,000

56,000

20,000

20,000Pseudocoelomate, unsegmented, bilaterally symmetrical worms;tubular digestive tract passing from mouth to anus; tiny; withoutcilia; live in great numbers in soil and aquatic sediments; some areimportant animal parasites

12,000Annelida(segmented worms)

Nematoda(roundworms)

Chordata(chordates)

Mollusca(mollusks)

Snails, clams,octopuses,nudibranchs

Arthropoda(arthropods)

Insects, crabs,spiders, millipedes

Typical Examples

TABLE 19.2 THE MAJOR ANIMAL PHYLA

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Jellyfish, hydra,corals, seaanemones

Soft, gelatinous, radially symmetrical bodies whose digestivecavity has a single opening; possess tentacles armed withstinging cells called cnidocytes that shoot sharp harpoons callednematocysts; almost entirely marine

Soft, gelatinous, radially symmetrical bodies whose digestivecavity has a single opening; possess tentacles armed withstinging cells called cnidocytes that shoot sharp harpoons callednematocysts; almost entirely marine

Asymmetrical bodies without distinct tissues or organs; saclikebody consists of two layers breached by many pores; internalcavity lined with food-filtering cells called choanocytes; mostmarine (150 species live in freshwater)

Microscopic, aquatic deuterostomes that form branchingcolonies, possess circular or U-shaped row of ciliated tentaclesfor feeding called a lophophore that usually protrudes throughpores in a hard exoskeleton; Bryozoa are also called Ectoproctabecause the anus, or proct, is external to the lophophore; marineor freshwater

ApproximateNumberofNamedSpecies

10,000

6,000

Key CharacteristicsTypical Examples

Porifera(sponges)

Echinodermata(echinoderms)

Cnidaria(cnidarians)

Lophophorates(Bryozoa, also calledmoss animalsorEctoprocta)

Bower bankia,Plumatella, seamats, sea moss

Rotifera(wheel animals)

Rotifers

Barrel sponges,boring sponges,basket sponges,vase sponges

Sea stars, seaurchins, sanddollars, seacucumbers

Phylum

5,150

4,000

2,000Small, aquatic pseudocoelomates with a crown of cilia around themouth resembling a wheel; almost all live in freshwater

TABLE 19.2 (CONTINUED)

SPONGES: ANIMALS WITHOUT TISSUES

• Sponges are members of the Phylum Porifera.• Their bodies a little more than masses of

specialized cells embedded in a gel-like matrix.• Clumps of cells disassociated from a sponge

can give rise to new sponges.

SPONGES: ANIMALS WITHOUT TISSUES

• The body of a sponge is perforated by many pores.• Choanocytes are flagellated cells that line the

body cavity of the sponge and draw in water through the pores.

• The sponge is a filter feeder which traps any food particles.

CNIDARIANS: TISSUES LEAD TO GREATER SPECIALIZATION

• Radiata are radially symmetrical and include two phyla.• Cnidaria comprises the hydra, jellyfish, corals,

and anemones.• Ctenophora comprises the comb jellies.

• The members of the Radiata have a body plan that allows them to interact with their environment on all sides.


• A major evolutionary advance in the Radiata is extracellular digestion of food.• Digestion begins outside of cells in a gut cavity

called the gastrovascular cavity.• This form of digestion allows animals to digest

an animal larger than itself.


• Cnidarians (phylum Cnidaria) are carnivores that capture prey with tentacles that ring their mouths.• These tentacles and, sometimes, the body surface bear

stinging cells called cnidocytes.• Within each cnidocyte is a harpoonlike barb, called a

nematocyst, which cnidarians use to spear prey and retract it towards the tentacle

• The nematocyst can discharge so explosively that it is capable of piercing the hard shell of a crab


• Cnidarians have two basic body forms:• Medusae are a

floating form.• Polyps are a sessile

form.

Epidermis

Gastrovascularcavity

Medusa

Mouth

GastrovascularcavityPolyp

Tentacles

Gastrodermis

Mesoglea


• Medusae are often called “jellyfish,” because of their gelatinous interior, or “stinging nettles,” because of their nematocysts.

• Polyps are pipe-shaped animals that usually attach to rock.• In corals, the polyps secrete a deposit of

calcium carbonate in which they live.

THE LIFE CYCLE OF OBELIA, A MARINE COLONIAL HYDROID

Eggs

Testis

Blastula

Medusae

Free-swimmingplanula larva

Settles down tostart new colony

Young colony andasexual budding

Maturecolony

Feedingpolyp

Ovary

Sexualreproduction

Sperm

Zygote

Reproductivepolyp

Medusa bud

SOLID WORMS: BILATERAL SYMMETRY

• Body symmetry differs among the Eumetazoa.• Radial symmetry

means that multiple planes cutting the organism in half will produce mirror images.

• Bilateral symmetry means that only one plane can cut the organism in half to produce mirror images.


• Most bilaterally symmetrical animals have evolved a definitive head end.• This process is termed cephalization.


• The bilaterally symmetrical eumetazoans produce three embryonic layers.• Ectoderm will develop into the outer coverings

of the body and the nervous system.• Mesoderm will develop into the skeleton and

muscles.• Endoderm will develop into the digestive

organs and intestine.


• The solid worms are the simplest of all bilaterally symmetrical animals.• The largest phylum of these worms is the Phylum Platyhelminthes, which includes the flatworms.• Flatworms lack any internal cavity other than

the digestive tract.• This solid condition is called acoelomate.

• Flatworms are the simplest animals in which organs occur.


• Most flatworms are parasitic but some are free-living.• Flatworms range in size from less than a

millimeter to many meters long.


• There are two classes of parasitic flatworms:• Flukes • Tapeworms

• The parasitic lifestyle has resulted in the eventual loss of features not used or needed by the parasite.• For example, parasitic flatworms do not need

eyespots.• This loss of features that lack adaptive purpose

for parasitism is sometimes called degenerative evolution.

LIFE CYCLE OF THE HUMAN LIVER FLUKE, CLONORCHIS SINENSIS

Flukes often require two or more hosts to complete their life cycles

12

3Metacercarialcysts in fishmuscle

Raw, infected fishis consumed byhumans or othermammals

Liver

Adult fluke

Egg containingmiracidium

Miracidium hatchesafter being eatenby snail

SporocystRedia

Cercaria

Bile duct


• Tapeworms are a classic example of degenerative evolution.• The body of a tapeworm has been reduced to

two primary functions.• Eating• Reproduction


• Those flatworms that have a digestive cavity, have an incomplete gut, one with only one opening.• The gut branches throughout the body and is

involved in both digestion and excretion.

• The parasitic flatworms lack a gut entirely and absorb food directly through their body walls.


• Flatworms lack a circulatory system and all cells must be within diffusion distance of oxygen and food.

• Flatworms have a simple nervous system, either a nerve net or longitudinal nerve cords with cross connections.• Free-living forms have

eyespots to distinguish light from dark.

ROUNDWORMS: THE EVOLUTION OF A BODY CAVITY

• A key transition in the evolution of the animal body plan was the evolution of the body cavity.

• The evolution of an internal body cavity helped improve the animal body design in three areas:• circulation• movement• organ function


• There are three basic kinds of body plans found in bilaterally symmetrical animals:• Acoelomates have no

body cavity.• Pseudocoelomates

have a body cavity (called a pseudocoel) located between the mesoderm and the endoderm.

• Coelomates have a body cavity (called a coelom) that develops entirely within the mesoderm.

Pseudocoelomate

Mesoderm

Ectoderm

Acoelomate

Coelomate

Endoderm

Pseudocoel

Ectoderm

Mesoderm

Coelomiccavity

Endoderm

Ectoderm

Mesoderm

Endoderm


• The largest pseudocoelomate phylum is Nematoda, containing about 20,000 species.• The members of this phylum include nematodes,

eelworms, and other roundworms.• Nematodes are unsegmented, cylindrical worms

covered by a flexible cuticle that is molted as they grow.

• Nematodes move in a whiplike fashion.

http://youtu.be/tp-O3LME3OU


• Some nematodes are parasitic in humans, cats, dogs, and animals of economic importance.• Heartworm in dogs is caused by a nematode• Trichinosis is an infection caused by the

nematode Trichinella and is transmitted to humans who eat undercooked pork.

• Intestinal roundworms, Ascaris lumbricoides, live in human intestines.


• Another pseudocoelomate phylum is Rotifera.• Rotifers are small,

aquatic organisms that have a crown of cilia at their heads.

• The cilia help in both locomotion and feeding.

http://youtu.be/cYNJOVDQexA

MOLLUSKS: COELOMATES

• The body of a mollusk (Phylum Mollusca) is comprised of three regions:• a head-foot • a visceral mass containing the body’s organs.• a mantle that envelopes the visceral mass and

is associated with the gills.


• There are three major groups of mollusks:• Gastropods—include

the snails and slugs.• Bivalves—include

clams, oysters, and scallops.

• Cephalopods—include the octopuses and squids.


• Mollusks have a unique feeding structure, called a radula.• The radula is a rasping tonguelike organ that bears rows

of pointed, backward-curving teeth.


• In most mollusks, the outer surface of the mantle secretes a protective shell.• The shell has multiple layers comprised of

protein, calcium, and pearl.

ANNELIDS: THE RISE OF SEGMENTATION

• One of the early innovations in body plan to arise among the coelomates was segmentation.• Segmentation is the building of a body from a

series of similar segments.• This body plan offers a lot of flexibility in that

small changes to segments can produce a new kind of segment with different functions.

• The first segmented animals to evolve were the annelid worms, Phylum Annelida.

ANNELIDS: THE RISE OF SEGMENTATION

• The basic body plan of an annelid is a tube within a tube.• The digestive tract is

suspended within the tube of the coelom.

• There are three body plan characteristics:• Repeated segments• Specialized segments• Connections

ARTHROPODS: ADVENT OF JOINTED APPENDAGES

• The most successful of all animal groups is the Phylum Arthropoda, consisting of the arthropods.• These animals have jointed appendages.• In addition to joints, arthropods have an

exoskeleton made of chitin.• The muscles of arthropods attach to the

interior of this outer shell.• The shell offers protection against predators

and water loss.


• Chitin cannot support much weight.• Arthropod size is limited as a result.

• Arthropod bodies are segmented like annelids.• Segments often fuse into functional groups in

the adult stage.

Head Thorax Abdomen

Antennae

Mouth


• Chelicerates are arthropods that lack jaws.• They include spiders, mites, scorpions, and

horseshoe crabs.• Mandibulates are arthropods with jaws,

called mandibles.• They include the crustaceans, insects, centipedes,

and millipedes. Antenna

Eye

Mandible

Eyes

Pedipalp

Chelicera


• The chelicerate fossil record goes back 630 million years.• A surviving type of chelicerate from this period

is the horseshoe crab.


• The class Arachnida has 57,000 named species of arachnids, including spiders, ticks, mites, scorpions, and daddy longlegs.• Arachnids have a pair of chelicerae, a pair of

pedipalps, and four pairs of walking legs.


• Crustaceans belong to the subphylum Crustacea and comprise a diverse group of mandibulates.• There a 35,000 species of

crustaceans described including species of crabs, shrimps, lobsters, crayfish, water fleas, pillbugs, and sowbugs.


• Most crustaceans have two pairs of antennae, three pairs of chewing appendages, and various numbers of legs.


• Crustaceans pass through a larval stage called the nauplius.


• Millipedes and centipedes have bodies that consist of a head region followed by numerous similar segments.• Centipedes have one

pair of legs per segment while millipedes have two.

• Centipedes are all carnivorous while millipedes are herbivorous.


• Insects belong to the Class Insecta and are the largest group of arthropods.• They are the most

abundant eukaryotes on the earth.

• Insects have three body sections:• Head• Thorax• Abdomen

PROTOSTOMES AND DEUTEROSTOMES

• There are two major kinds of coelomate animals representing two distinct evolutionary lines:• Protostomes

• The mouth develops from or near the blastopore.• Deuterostomes

• The anus forms from or near the blastopore; the mouth forms on another part of the blastula. CoelomAnusCoelomArchenteronMesoderm

Mouth

Mouth forms fromblastopore

Mouth

Coelom

AnusAnus formsfrom blastopore

Archenteronoutpockets toform coelom

BlastoporeBlastula32 cells16 cells8 cells4 cells2 cells1 cell

Deuterostomes

1 cell

Protostomes

2 cells 4 cells 8 cells 16 cells

CoelomArchenteron

Mesoderm splitsBlastoporeBlastula32 cells

PROTOSTOMES AND DEUTEROSTOMES

• Deuterostomes also differ from protostomes in three other fundamental ways:• The pattern of cleavage

• Protostomes have spiral cleavage while deuterostomes have radial cleavage.

• Fating of cells• Occurs later in deuterostome cleavage than in

protostome cleavage.• Origin of the coelom.

ECHINODERMS: THE FIRST DEUTEROSTOMES

• Echinoderms are deuterostomes that belong to the phylum Echinodermata.• Echinoderm means “spiny skin”

and refers to the endoskeleton of calcium-rich ossicles just beneath the echinoderm’s skin.

• Entirely marine animals and include sea stars, sea urchins, sand dollars, and sea cucumbers.

• All are bilaterally symmetrical as larvae but become radially symmetrical as adults.

ECHINODERMS: THE FIRST DEUTEROSTOMES

• A key adaptation of echinoderms is the water vascular system that aids movement.• This system is fluid-filled

and composed of a central ring canal from which five radial canals extend out into the arms.

• From each radial canal, tiny vessels extend through short side branches into thousands of tiny, hollow tube feet.

• Echinoderms can extend the tube feet, attach them to the ocean floor, and pull against them to move.

http://youtu.be/8JOxiT5_zpc

CHORDATES: IMPROVING THE SKELETON

• Chordates are deuterostomes that belong to the phylum Chordata.• They exhibit a truly internal endoskeleton with

muscles attached to an internal rod, called a notochord.

• This innovation opened the door to large body sizes not possible in earlier animal forms.


• The approximately 56,000 species of chordates share four principal features:• Notochord• Nerve cord• Pharyngeal pouches• Postanal tail

• All chordates have all four of these characteristics at some time in their lives.


• Not all chordates are vertebrates.• Tunicates and lancelets are chordates.


• Vertebrate chordates differ from tunicates and lancelets in two important respects.• Vertebrates have a

backbone that replaces the role of the notochord.

• Vertebrates have a distinct and well-differentiated head.

chapter 19 evolution of the animal phyla. general features of animals animals share many important...

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animal family tree animals

animal phyla

animal family tree eumetazoans

body plan

traditional animal phylogeny

family trees

bilateral symmetry

body mass