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.