23.2 Animal Diversity
KEY CONCEPT
More than 95 percent of all animal species are
invertebrates.
23.2 Animal Diversity
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.
23.2 Animal Diversity
• 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
tail
head
23.2 Animal Diversity
Animals are grouped using a variety of criteria.
• Three criteria are used to
categorize animals.
– body plan symmetry
– tissue layers
– developmental patterns
gastrovascular
cavity
mouth
mesoglea
brain
muscle
hearts
blood vessels
oral arms
mouth
segment
digestive track
nerve cord
tentacles
23.2 Animal Diversity
• 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.
23.2 Animal Diversity
• 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.
23.2 Animal Diversity
• 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 mouthfirst, and anus second.
– Deuterostomes first form the
anus and then the mouth.
23.2 Animal Diversity
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.
NO TISSUES
RADIAL
lancelets,
vertebrates
sea stars,
sea urchins
crustaceans,
insects, spiders
Echinodermata
Nematoda Arthropoda
Chordata
roundworms
clams, snails,
octopuses
Segmented
worms
flatworms
Cnidaria Platyhelminthes Annelida
Mollusca
jellyfish, coral, anemones
sponges
Porifera
DUETEROSTOMES
PROTOSTOMES
23.2 Animal Diversity
• The current organization of the animal kingdom shows
some unexpected relationships.
• Technological advancements
help to clarify evolutionary
relationships.