Biology Now, 2e

306 ■ CHAPTER 17 Animals and Human Evolution

BIODIVERSITY

Figure 17.5

Segmentation in animals

Segmentation, a body plan in which segments repeat, enabled the evolution of diverse uses of

appendages, shown here in a lobster.

Q1: List all of the lobster’s thoracic appendages.

Q2: Which of the lobster’s appendages is most important for sensing the environment?

Q3: What body segments do you (a human) have?

Crusher claw

Tail segments

Walking legs

Swimming

legs

Cutting claw

Head

Abdomen Thorax

Eye

Antennae

The head appendages include

antennae, which are touch-

sensitive sensory organs, and

mouthparts used in feeding.

This thoracic

appendage is used for

crushing shelled prey.

Watch out for this thoracic

appendage when handling a live

lobster! It can be moved very

swiftly and is used for cutting

up food and also in defense.

These thoracic

appendages are

used for walking

and burrowing.

These abdominal

appendages are

used for swimming.

or in water. Locomotion is a key evolutionary

innovation of animals and one that has sparked

a wide range of behaviors, including varied ways

of capturing prey, eating prey, avoiding being

captured, attracting mates, caring for young,

and migrating to new habitats.

Many animals have segmented bodies; that

is, their body plan consists of repeated units

known as segments (Figure 17.5). Specialized

body parts, known as appendages, often origi-

nate in pairs from specific segments of the body,

such as fins or limbs. Over evolutionary time, the

segments and the appendages that spring from

them have evolved diverse form and function,

enabling the animal body to adapt to new habi-

tats or acquire new modes of life.

The evolution of just the posterior segments

of arthropods illustrates how evolution can take

a basic body plan and modify it to produce many

variations over time: The last segment in the

body of arthropods has evolved into the delicate

abdomen of the butterfly, the piercing abdomen

of the wasp, and the delicious tail of the lobster.

The front appendage of vertebrates has evolved

as an arm in humans, a wing in birds, a flipper

in whales, an almost nonexistent nub in snakes,

and a front leg in salamanders and lizards.

Mammals R Us

To get back to our egocentric focus—that is,

humans—we are part of the kingdom Animalia

and the class Mammalia. Humans share specific

features with all other mammals, including

body hair, sweat glands, and milk produced by

mammary glands. And at the risk of tooting our

own horn, mammals have been a high ly success-

ful class of animals, with over 5,000 species living

in a variety of habitats. This is largely thanks to