•18.11 Arthropods are segmented animals with jointed appendages and an exoskeleton

– Various adaptations have made arthropods the most successful animals on Earth

• Jointed appendages adapted for different functions

• Exoskeleton: external skeleton that protects and provides attachment points for muscles

• Distinct groups of segments: head, thorax, abdomen

• Open circulatory system

LE 18-11a

Cephalothorax Abdomen

Head Thorax

Antennae(sensoryreception)

Head

Pincer (defense) Mouthparts (feeding)

Walking legs

Swimmingappendages

– Chelicerates• Horseshoe crabs• Arachnids, such as spiders, scorpions, mites, and

ticks• Most live on land and are carnivores

– Millipedes and centipedes• Have similar segments over most of body• Segments bear two pairs (millipedes) or one pair

(centipedes) of legs

– Crustaceans• Nearly all are aquatic• Include lobsters, crabs, shrimps, barnacles,

small copepods, and krill

•18.12 Insects are the most diverse group of organisms

– Insects have a number of common features• Three-part body consisting of head, thorax, and

abdomen– Head usually has antennae and eyes

• Mouthparts specialized for particular kinds of eating• Three sets of legs• One or two pairs of wings (most, but not all)

LE 18-12a

Head Thorax

Hindwing

Antenna

Eye

Mouthparts

Abdomen

Forewing

•18.13 Echinoderms have spiny skin, an endoskeleton, and a water vascular system for movement

– Echinoderms, phylum Echinodermata, are slow-moving or sessile marine animals

• Include organisms such as sea stars, sand dollars, and sea urchins

• Adults are radially symmetrical• External bumps or spines are extension of hard

endoskeleton

– The water vascular system is unique to echinoderms

• Network of water-filled canals that branch into tube feet

• Suction cup–like tube feet used for feeding, gas exchange, and locomotion

– Embryonic features such as bilateral symmetry places echinoderms on the same clade as chordates

LE 18-13a

Stomach

Anus

Canals

Tube feet

Spines

LE 18-13b

Tube foot

LE 18-13c

Tube foot

Spine

•18.14 Our own phylum, Chordata, is distinguished by four features

– Four distinctive features appear in the embryos, and often in the adults, of chordates, phylum Chordata:

• A dorsal, hollow nerve cord• A stiff notochord• Pharyngeal slits• A muscular post-anal tail

– The simplest chordates are tunicates and lancelets

• Marine invertebrates that use pharyngeal slits for suspension feeding

• Tunicates: small, saclike; adhere to rocks and reefs; likely represent the deepest branch of the chordate lineage

• Lancelets: small, bladelike; live in marine sands; closest living relatives of the vertebrates

LE 18-15

Ancestral chordate

Head

Lungs or lung derivatives

Vertebral column

Brain

Jaws

Lobed fins

Legs

Amniotic egg

Milk

Mam

mal

s

Rep

tile

s

Am

ph

ibia

ns

Lo

be-

fin

s

Ray

-fin

ned

fis

hes

Sh

arks

, ra

ys

Lam

pre

ys

Hag

fish

es

Lan

cele

ts

Tu

nic

ates

Chordates

Craniates

Vertebrates

Jawed vertebrates

Tetrapods

Amniotes