•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