1. Apply Concepts Design a “new” invertebrate. Create and illustration on which you point out its body plan features. Then show its place on the cladogram of invertebrates and write a caption explaining how its features helped you decide where it belongs.

CH 26 ANIMAL EVOLUTION AND DIVERSITY

26.1 Invertebrate Evolution and Diversity

Fossil evidence indicates that the first animals began evolving long before the Cambrian Explosion.

Origins of the Invertebrates

Roughly 3 billion years after the first prokaryotic cells evolved, all prokaryotes and eukaryotes were single-celled

Animals evolved from ancestors they shared with organisms called choanoflagellates Single-celled eukaryotes that sometimes grow in

colonies Similar to sponges.

Traces of Early Animals

Oldest evidence of multicellular life comes from microscopic fossils that are roughly 600 million years old

First animals were tiny and soft-bodied, so few fossilized bodies exist.

Ediacaran Fauna

Before the Cambrian Period From the Ediacara Hills of Australia Fossils 565 to about 544 million years old Flat and lived on the bottom of shallow seas Little evidence of cell, tissue, or organ

specialization, and no organization into a front and back end.

Cambrian Explosion

Cambrian Period began about 542 million years ago

Burgess Shale of Canada Fossils show that over a period of 10–15 million

years, animals evolved complex body plans, including specialized cells, tissues, and organs.

By the end of the Cambrian Period, all the basic body plans of modern phyla had been established

Later evolutionary changes, which produced the more familiar body structures of modern animals, involved variations on these basic body plans.

Invertebrates are the most abundant animals on Earth.

Sponges

Phylum: Porifera Most ancient members of the kingdom Animalia Multicellular, heterotrophic, lack cell walls, and

contain a few specialized cells.

Cnidarians

Phylum: Cnidaria Includes jellyfishes, sea fans, sea anemones,

hydras, and corals Aquatic, soft-bodied, carnivorous, radially

symmetrical animals with stinging tentacles arranged in circles around their mouths.

Arthropods

Phylum: Arthropoda Includes spiders, centipedes, insects, and

crustaceans Bodies divided into segments, a tough external

skeleton called an exoskeleton, cephalization, and jointed appendages (legs and antennae)

Appeared in the sea about 600 million years ago.

Nematodes (Roundworms)

Phylum: Nematoda Unsegmented worm with pseudocoeloms,

specialized tissues and organ systems, and digestive tracts with two openings

More closely related to the arthropods than flatworms.

Flatworms

Phylum: Platyhelminthes Soft, unsegmented, flattened worms that have

tissues and internal organ systems Simplest animals to have three embryonic germ

layers, bilateral symmetry, and cephalization No coelms.

Annelids

Phylum: Annelida Includes earthworms, some marine worms, and

leeches Worms with segmented bodies and a true coelom.

Mollusks

Phylum: Mollusca Includes snails, slugs, clams, squids, and octopi Soft-bodied animals that have an internal or

external shell Have true coeloms and complex organ systems May have a trochophore

Immature free-swimming larva stage.

Echinoderms

Phylum: Echinodermata Includes sea stars, sea urchins, and sand dollars Have spiny skin and an internal skeleton Have a water vascular system that include suction-

cuplike tube feet Deuterostomes.