Invertebrates evolutionary history

• Animals evolved from ancestors they shared with organisms called choanoflagellates
• Choanoflagellates share several characteristics with sponges, the simplest multicellular animals
• Other fossils from this time period have been tentatively identified as parts of sponges and animals to jellyfish
• Some important discoveries about invertebrate life before the Cambrian Period come from fossils in the Ediacara Hills of Australia
• Fossils from roughly 565 to about 544 million years ago show body plans that are different from those of anything alive today
• Many of the organisms in the Ediacaran fauna were flat and lived on the bottom of shallow seas
• Cambrian fossils show that over a period of 10-15 million years, animals evolved complex body plans, including specialized cells, tissues, and organs
• A number of Cambrian fossils have been identified as ancient members of modern invertebrate phyla, such as the fossil of arthropod Marrella spledens
• Today, invertebrates are the most abundant animals on Earth
• Invertebrates live in nearly every ecosystem, participate in nearly every food web, and vastly outnumber so-called “higher animals”, such as reptiles and mammals
• The Cladogram shows current hypotheses of evolutionary relationships among modern invertebrates
• Modern sponges and cnidarians have little internal specialization
• All invertebrates except sponges exhibit some type of body symmetry
• Concentration of sense organs and nerve cells in the front of the body is known as cephalization
• Most complex animal phyla have true coeloms that are lined completely with mesoderm
• In most invertebrates, the zygote divides repeatedly to form a blastula - a hollow ball of cells
• Sponges are the most ancient members of the Kingdom Animalia
• Cnidarians include jellyfishes, sea fans, sea anemones, hydras, and corals
• Flatworms are the simplest animals to have three embryonic germ layers, bilateral symmetry, and cephalization
• Mollusks are soft-bodied animals that have true coeloms surrounded by mesoderm and complex organ systems
• Annelids are worms with segmented bodies and a true coelom lined with tissue derived from mesoderm
• Nematodes are unsegmented worms with pseudocoeloms, specialized tissues and organ systems, and digestive tracts with two openings - a mouth and an anus
• Arthropods possess an exoskeleton with a cuticle made of chitin, have a segmented body, and jointed limbs
• Echinoderms include sea stars, sea urchins, and sand dollars, have spiny skin and internal skeletons, and exhibit five-part radial symmetry
• Some organisms like worms and arthropods, nerve cells are arranged in structures known as ganglia
• In more complex invertebrates, such as certain mollusks, nerve cells form an organ called brain.
• SPONGES - Phylum Porifera - Sponges are the most ancient members of the Kingdom Animalia. - They are multicellular, heterotrophic, lack cell walls, and contain a few specialized cells.
• CNIDARIANS - Phylum Cnidaria- includes jellyfishes, sea fans, sea anemomes, hydras and corals - Cnidarians are aquatic, soft- bodied, carnivorous, radially symmetrical animals with stinging tentacles arranged in circles around their mouths. - They are the simplest animals to have body symmetry.
• FLATWORMS - Phylum Platyhelminthes - Flatworms are soft, unsegmented, flattened worms that have tissues and internal organ systems. - They are the simplest animals to have three embryonic germ layers, bilateral symmetry and cephalization. - Flatworms do not have coeloms
• MOLLUSKS - Phylum Mollusca- includes snails, slugs, clams, squids and octopi - Mollusks are soft-bodied animals that have an internal or external shell. - They have true coeloms surrounded by mesoderm and complex organ systems - Many mollusks have a free- swimming larva, or immature stage called a trochopore.
• ANNELIDS - Phylum Annelida- includes earthworms, some marine worms and leeches - Annelids are worms with segmented bodies and a true coelom line with tissue derived from mesoderm
• NEMATODES - Phylum Nematoda - Unsegmented worms with pseudocoeloms, specialized tissues and organ systems, and digestive tracts with two openings-a mouth and an anus. - Nematodes were once thought to be closely related to flatworms, annelids and mollusks but have been found to more closely related to arthropods.
• ARTHROPODS - Phylum Arthropoda - Possess an exoskeleton with a cuticle made of chitin which is being shed periodically - Have a segmented body, jointed limbs - Includes insects, crustaceans, spiders,
• ECHINODERMS - Phylum Echinodermata- includes sea stars, sea urchins and sand dollars - Echinoderms have spiny skin and internal skeletons - They also have a water vascular system- a network of water-filled tubes that include suction-cuplike tube feet, which are used for walking and gripping prey. - Most exhibit five-part radial symmetry and are deuterostomes.
• In protostomes, the first opening in the embryo becomes the mouth. In deuterostomes, the first opening in the embryo becomes the anus, and the mouth develops later.
• Acoelomates: These organisms, like flatworms, do not have a body cavity. Their space between the body wall and the digestive tract is filled with tissues. Pseudocoelomates: These organisms, like roundworms, have a body cavity called a pseudocoelom, which is not completely lined by mesoderm.