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

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.

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

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

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.

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.

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.

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,

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