Biology Now, 2e

304 ■ CHAPTER 17 Animals and Human Evolution

BIODIVERSITY

artificial grouping (like that of protists) of animals

with varied evolutionary histories and different

sets of evolutionary adaptations.

Body plan is another key factor that distin-

guishes one group of animals from another. All

animals except sponges, the most ancient of the

animal lineages, have a distinct body symmetry.

The asymmetric sponges aside, animals can be

divided into two main groups: those with radial

“Get a Backbone!” on page 305). The phylum

also includes several subgroups of less familiar

animals, such as sea squirts and lancelets, that

have a nerve cord along the back of the body but

no backbone. Chordates that do possess a back-

bone are known as vertebrates. All other phyla of

animals are informally lumped together as inver-

tebrates. But keep in mind that “invertebrates”

is not an evolutionarily meaningful label; it is an

Figure 17.03

03/24/17

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BIONOW2

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Lancelets

Lampreys

Hagfishes

Sharks

and rays

Ray-finned

fishes

Coelacanths

Amphibians

Reptiles

and birds

Mammals

Sea

squirts

Skull

Notochord

Backbone

Jaws

Scales

Bony skeleton

Lobed fins

Four limbs

Amniotic egg

Fur, mammary glands

Figure 17.3

Not all chordates have a backbone

As with all eukaryotes except the protists, the evolutionary relatedness of the chordates is very well

known. Not all chordates have a backbone, but all have a notochord, which is the precursor to part of

the vertebrate backbone.

Q1: Do amphibians have amniotic eggs?

Q2: What group of animals has jaws but not a bony skeleton?

Q3: When people talk about animals, they are sometimes referring only to mammals. How would

you explain to them their error?