Chapter 17Chapter 17

The Evolution of Animals

What Is an Animal?– Animals are:• Eukaryotic• Multicellular• Heterotrophic organisms that obtain nutrients by ingestion• Able to digest their food within their bodies

– Animal cells lack the cell walls that provide strong support in the bodies of plants and fungi.

Ancestralprotist

No true tissues

Radial symmetry

Tissues

Bilateral symmetry

Sponges

Cnidarians

Molluscs

Flatworms

Annelids

Roundworms

Arthropods

Echinoderms

Chordates

Figure 17.5

– A second major evolutionary split is based on body symmetry.• Radial symmetry refers to animals that are identical all around a

central axis.• Bilateral symmetry exists where there is only one way to split the

animal into equal halves.

Radial symmetry. Parts radiate from the center, so any slicethrough the central axis divides into mirror images.

Bilateral symmetry. Only one slice can divide left and rightsides into mirror-image halves.

Figure 17.6

– Animals also vary according to the presence and type of body cavity, a fluid-filled space separating the digestive tract from the outer body wall.

– There are differences in how the body cavity forms.• If the body cavity is not completely lined by tissue derived

from mesoderm, it is a pseudocoelom.• A true coelom is completely lined by tissue derived from

mesoderm.

(a) No body cavity

(b) Pseudocoelom

(c) True coelom

Body covering(from ectoderm)

Tissue-filledregion (frommesoderm)

Body covering(from ectoderm)

Body covering(from ectoderm)

Musclelayer (frommesoderm)

Tissue layer liningcoelom andsuspendinginternal organs(from mesoderm)

Digestive tract(from endoderm)

Digestive tract(from endoderm)

Digestive tract(from endoderm)

Pseudocoelom

Coelom

Figure 17.7

MAJOR INVERTEBRATE PHYLA– Invertebrates:• Are animals without backbones• Represent 95% of the animal kingdom

Sponges– Sponges include sessile animals that lack true tissues

and that were once believed to be plants.

Figure 17.8b

Cnidarians– Cnidarians (phylum Cnidaria) are characterized by:• The presence of body tissues • Radial symmetry • Tentacles with stinging cells

– The basic body plan of a cnidarian is a sac with a gastrovascular cavity, a central digestive compartment with only one opening.

– The body plan has two variations: • The sessile polyp • The floating medusa

– Cnidarians are carnivores that use tentacles, armed with nematocysts (or cnidocytes) (“stinging cells”), to capture prey.

Mouth/anus

Tentacle

Polyp form

Gastrovascularcavity

Coral Hydra

Sea anemone

Gastrovascularcavity

Mouth/anusTentacle

Medusa form

JellyFigure 17.9

Tentacle

Trigger

Capsule

Cnidocyte

Dischargeof thread

Prey

Coiledthread

Figure 17.10

Molluscs– Molluscs (phylum Mollusca) are represented by soft-

bodied animals, usually protected by a hard shell.– Many molluscs feed by using a file-like organ called a

radula to scrape up food.– The body of a mollusc has three main parts: • A muscular foot used for movement• A visceral mass housing most of the internal organs• A mantle, which secretes the shell if present

Visceral mass

Reproductiveorgans

Digestivetract

Mantlecavity

Nervecords

Digestivetract

Coelom

Radula

Radula

Kidney Heart

Shell

Mouth

Mouth

Anus

Gill

Foot

Mantle

Figure 17.11

– The three major groups of molluscs are:• Gastropods, protected by a single, spiraled shell

Gastropods

Snail (spiraled shell)

Sea slug (no shell)Figure 17.12a

• Bivalves, with a shell divided into two halves hinged together

Bivalves(hinged shell)

ScallopFigure 17.12b

• And cephalopods– Typically lacking an external shell – Built for speed and agility

Cephalopods(large brain and tentacles)

Octopus Squid

Figure 17.12c

Gastropods Bivalves(hinged shell)

Cephalopods(large brain and tentacles)

Snail (spiraled shell)

Sea slug (no shell)

Scallop Octopus Squid

MAJOR GROUPS OF MOLLUSCS

Figure 17.12

Flatworms– Flatworms (phylum Platyhelminthes) are the

simplest bilateral animals.– Flatworms include forms that are:• Parasites or• Free-living in marine, freshwater, or damp habitats

Digestive tract(gastrovascularcavity) Nerve cords

MouthEyespots(detect light)

Nervous tissueclusters(simple brain)

Planarian Bilateral symmetry

Blood fluke

Head Hooks

Suckers

Reproductive unitwith skin removed

Tapeworm

Figure 17.13

Annelids– Annelids (phylum Annelida) have:• Body segmentation, a subdivision of the body along its

length into a series of repeated parts• A coelom• A complete digestive tract with

– Two openings, a mouth and anus– One-way movement of food

Mouth

Brain

Coelom

Nerve cord

Blood vesselsWaste disposal organ

Anus

Accessoryhearts

Mainheart

Digestivetract

Segmentwalls

Figure 17.15

– The three main groups of annelids are:• Earthworms, which eat their way through soil• Polychaetes, marine worms with segmental appendages

for movement and gas exchange• Leeches, typically free-living carnivores but with some

bloodsucking forms

Earthworms Polychaetes Leeches

MAJOR GROUPS OF ANNELIDS

Christmas tree wormGiant Australian earthworm European freshwater leech

Figure 17.14

Roundworms– Roundworms (phylum Nematoda) are:• Cylindrical in shape, tapered at both ends• The most diverse and widespread of all animals

– Roundworms (also called nematodes) are:• Important decomposers • Dangerous parasites in plants, humans, and other animals

(a) A free-livingroundworm

(b) Parasiticroundworms in pork

(c) Canine heartInfected with parasiticroundworms

Figure 17.16

Arthropods– Arthropods (phylum Arthropoda) are named for

their jointed appendages.– There are about one million arthropod species

identified, mostly insects.– Arthropods are a very diverse and successful group,

occurring in nearly all habitats in the biosphere.– There are four main groups of arthropods.

MAJOR GROUPS OF ARTHROPODS

Arachnids

Crustaceans

Millipedes and Centipedes

Insects

Figure 17.17

General Characteristics of Arthropods

– Arthropods are segmented animals with specialized segments and appendages for an efficient division of labor among body regions.

– The body of arthropods is completely covered by an exoskeleton, an external skeleton that provides:• Protection • Points of attachment for the muscles that move

appendages

AbdomenCephalothorax

(head and thorax)

Swimmingappendage

Antenna(sensory reception)

Eyes onmovable stalks

Mouthparts (feeding)

Walking leg

Walking legs

Figure 17.18

– Arachnids:• Live on land• Usually have four pairs of walking legs and a specialized

pair of feeding appendages• Include spiders, scorpions, ticks, and mites

Arachnids

Two feedingappendages

Leg (four pairs)

Black widow spider Wood tick

Dust miteScorpion

Figure 17.19

– Crustaceans:• Are nearly all aquatic• Have multiple pairs of specialized appendages• Include crabs, lobsters, crayfish, shrimps, and barnacles

Crustaceans

Two feedingappendages Leg (three or more pairs)

Antennae

BarnaclesCrayfish

Pill bugShrimp

Crab

Figure 17.20

– Millipedes and centipedes have similar segments over most of the body.

– Millipedes:• Eat decaying plant matter • Have two pairs of short legs per body segment

– Centipedes:• Are terrestrial carnivores with poison claws • Have one pair of short legs per body segment

Millipedes and Centipedes

One pair of legs per segment

Two pairs of legsper segment

Millipede Centipede

Figure 17.21

– Insects typically have a three-part body:• Head• Thorax• Abdomen

– The insect head usually bears:• A pair of sensory antennae • A pair of eyes

– The mouthparts are adapted for particular kinds of eating.

– Flight is one key to the great success of insects.

Insect Anatomy

Antenna

Head Thorax Abdomen

Eye

Mouthparts

Figure 17.22

– Insects outnumber all other forms of life combined.– Insects live in:• Almost every terrestrial habitat • Freshwater• The air

Insect Diversity

Banded OrangeHeliconian

PrayingmantisGiraffe weevil

Yellow jacket wasp Leaf beetle

Leaf roller

Peacock katydid

Longhorn beetleFigure 17.23

– Many insects undergo metamorphosis in their development.

– Young insects may:• Appear to be smaller forms of the adult or• Change from a larval form to something much different as an adult

The larva (caterpillar) spendsits time eating and growing,molting as it grows.

After several molts, thelarva becomes a pupaencased in a cocoon.

Within the pupa, the larval organs breakdown and adult organs develop fromcells that were dormant in the larva.

Finally, the adult emergesfrom the cocoon.

The butterfly flies off and reproduces, nourished mainlyby calories stored when it was a caterpillar.

Figure 17.24-5

Monarch butterfliesFigure 17.24a

Echinoderms– Echinoderms (phylum Echinodermata):• Lack body segments• Typically show radial symmetry as adults but bilateral

symmetry as larvae • Have an endoskeleton• Have a water vascular system that facilitates movement

and gas exchange

– Echinoderms are a very diverse group.

Sea star

Sea urchin

Sea cucumber Sand dollar

Tube feet

Figure 17.25