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
headtail
fruit fly genes
human HOX-B genes
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
mouthmesoglea
oral arms
tentacles
blood vessels
brain
heartsmuscle
segment nerve cord
mouth
digestive track
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 animalinto mirror-image sides.
23.2 Animal Diversity
– radial symmetry: body arranged in circle around a central axis
• There are two types of body plan symmetry.– bilateral symmetry: body divides equally along one plane
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 mouth-
first, 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.
Porifera Cnidaria Platyhelminthes AnnelidaMollusca Nematoda Arthropoda
EchinodermataChordata
RADIAL
NO TISSUES
PROTOSTOMESDUETEROSTOMES
jelly
fish,
cor
al, a
nem
ones
spon
ges
flatw
orm
s
Seg
men
ted
wor
ms
clam
s, s
nails
,oc
topu
ses
roun
dwor
ms
crus
tace
ans,
inse
cts,
spi
ders
sea
star
s,
sea
urch
ins
lanc
elet
s,ve
rtebr
ates
23.2 Animal Diversity
• The current organization of the animal kingdom shows some unexpected relationships.
• Technological advancementshelp to clarify evolutionaryrelationships.