1. KEY CONCEPT More than 95 percent of all animal species are invertebrates.

2. Each animal phylum has a unique body plan.
Vertebrates have an internal segmented backbone.
Invertebrates do not have a backbone.
Invertebrates encompass most animal groups.

3. Hox genes tell embryonic cells which body part to become.
Differences in body plans result from differences in the expression of Hox genes.
Hox genes tell embryonic cells which body part to become.
Mutations in Hox genes led to the vast diversity of animal species.
head
tail
fruit fly genes
human
HOX-B genes

4. Animals are grouped using a variety of criteria.
Three criteria are used to categorize animals.
body plan symmetry
gastrovascular cavity
mouth
mesoglea
oral arms
tentacles
tissue layers
developmental patterns
blood vessels
brain
hearts
muscle
segment
nerve cord
mouth
digestive track

5. There are two types of body plan symmetry.
bilateral symmetry: body divides equally along one plane
Animals with bilateral symmetry can be divided equally along only one plane, which splits an animal
into mirror-image sides.

6. There are two types of body plan symmetry.
bilateral symmetry: body divides equally along one plane
radial symmetry: body arranged in circle around a central axis
Animals with radial symmetry have body parts arranged in a circle around a central axis.

7. Bilateral animals have three distinct layers of tissue; radial animals have only two.
both animal types have ectoderm and endoderm
bilateral animals have mesoderm
Animals are divided into two major groups, the protostomes and the deuterostomes.
Protostomes form mouth-first, and anus second.
Deuterostomes first form the anus and then the mouth.

8. A comparison of structure and genetics reveals the evolutionary history of animals.
Protostomes and deuterostomes are the two major radiations on the animal phylogenetic tree.
Porifera
Cnidaria
Platyhelminthes
Annelida
Mollusca
Nematoda
Arthropoda
Echinodermata
Chordata
RADIAL
NO TISSUES
PROTOSTOMES
DUETEROSTOMES
jellyfish, coral, anemones
sponges
flatworms
Segmented
worms
clams, snails,
octopuses
roundworms
insects, spiders
crustaceans,
sea stars,
sea urchins
vertebrates
lancelets,

9. The current organization of the animal kingdom shows some unexpected relationships.
Technological advancements help to clarify evolutionary relationships.