Biology Now, 2e

Neanderthal Sex ■ 305

symmetry and those with bilateral symmetry.

The body of an animal with radial symme-

try (Figure 17.4, left)—including cnidarians

like jellyfish, sea anemones, and corals—can

be sliced symmetrically along any number of

vertical planes that pass through the center of

the animal, like cutting a pie. Radial symmetry

gives an animal sweeping, 360-degree access

to its environment. The animal can snare food

drifting in from any direction of the compass,

and it can also sense and respond to danger from

any side.

Animals with bilateral symmetry (Fig -

ure 17.4, right), on the other hand, can be

divided by just one plane passing vertically from

the top to the bottom of the animal into two

halves that mirror each other. Bilateral animals,

which include all chordates, have distinct right

and left sides, with near-identical body parts

on each side. The symmetrical arrangement of

body parts on either side of a central body facili-

tates movement in bilateral animals. The paired

arrangement of limbs or fins, for example,

enables quick and efficient movement on land

Get a Backbone!

W

ithin the kingdom Animalia, humans belong to the

phylum of chordates and the subphylum of vertebrates.

All chordates have a dorsal notochord, a flexible rod along

the center of the body that is critical for development. In

vertebrates, the dorsal notochord has evolved to become

the cushioning discs between vertebrae (singular “vertebra”),

which are strong, hollow sections of the backbone, or

vertebral column. Vertebrates include fishes, amphibians

(frogs and salamanders), reptiles (snakes, lizards, turtles,

and crocodiles), birds, and mammals.

The jawless fishes were the first vertebrates to evolve.

Their skeletons—including the backbone—were made from

a strong but flexible tissue called cartilage. Only a few

groups of jawless fishes have survived to the present day,

most notably the lampreys. The next great leap in vertebrate

evolution was hinged jaws, which enabled predators to grab

and swallow prey efficiently. The evolution of teeth made

jaws even more effective because teeth enabled animals to

seize and tear food.

Another major step in the evolution of vertebrates was

the replacement of the cartilage-based skeleton with a

denser tissue strengthened by calcium salts: bone. Although

the descendants of cartilaginous fishes—sharks, skates,

and rays—are still with us today, bony fishes are far more

diversified and widespread in both marine and freshwater

environments. With more than 30,000 species, bony fishes

are the most diverse vertebrates today.

The advent of lungs was a crucial milestone in the

transition of vertebrates onto land. Amphibians made this

transition only partially; they can live on land but must return

to the water to lay eggs and breed. The several thousand

species of amphibians include frogs and salamanders.

Reptiles were the first vertebrates to head into drier

environments, and they evolved a number of adaptive traits

to deal with the risk of dehydration. These adaptations

included skin covered in waterproof scales, a water-

conserving excretory system, and the amniotic egg with its

calcium-rich protective shell, which retards moisture loss

while allowing the entry of life-giving oxygen and the release

of waste carbon dioxide for the developing embryo. Reptiles

dominated Earth during the age of the dinosaurs, and the

dinosaurs’ descendants (as we saw in Chapter 14) remain

with us today in the form of birds. Like mammals, birds are

warm-blooded, but they have feathers instead of fur for

insulation. At least 10,000 different species of birds are

living today.

Posterior

end

Anterior

end

Radial symmetry Bilateral symmetry

Sea anemone

Planes of

symmetry

Tuna

Dorsal side

Ventral side

Figure 17.4

Body symmetry in animals

All animals other than sponges have symmetrical bodies that enable them to

better sense and respond to the world around them.

Q1: Is a sea star radially or bilaterally symmetrical?

Q2: What advantage might a bilaterally symmetrical animal have over

one that is radially symmetrical, and vice versa?

Q3: What kind of symmetry do you (a human) have?