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Invertebrates. A Survey of Invertebrates. Trends in Invertebrate Evolution. Common ancestors of multicellular animals had already evolved two distinct cell layers called germ layers , separated by a jelly-like middle layer Refer to chart on embryonic development. Embryonic Development.

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  1. Invertebrates A Survey of Invertebrates

  2. Trends in Invertebrate Evolution • Common ancestors of multicellular animals had already evolved two distinct cell layers called germ layers, separated by a jelly-like middle layer • Refer to chart on embryonic development

  3. Embryonic Development • Egg + sperm  zygote • Zygote undergoes cleavage (mitosis) to form a mass of cells • Mass develops or several days, hollow space develops in the middle • At this point, it is called a blastula

  4. Embryonic Development • Blastula further develops into gastrula, at which time three germ layers are formed • Germ layers will each develop into different tissues in the animal

  5. Germ Layers • The ectoderm (outer layer) will develop into skin and other body coverings, glands, and nervous system tissues • The endoderm (inner layer) grows into tissues and organs of the digestive tract and respiratory system

  6. Germ Layers • Middle layers varies by animal • Sponges and jellyfish only have a primitive layer of jelly-like cells • Higher animals have an actual third cell layer called the mesoderm which develops into skeletal, muscle, and circulatory tissues, and reproductive system

  7. Trends in Invertebrate Evolution • Second trend – existence of mesoderm-lined cavity called coelom • Provides space for organs to grow and function without being twisted or squeezed by body movements • Fluid within coelom helps to carry food, wastes, dissolved gases

  8. Coelom • Acoelomates = animals without a coelom, like the hydra • Pseudocoelomates = have a body cavity between mesoderm and endoderm, like roundworm • Coelomates = have a true coelom that houses digestive tract and organs, like annelids (earthworm)

  9. Trends in Invertebrate Evolution • Third trend is the evolution of a body plan, built of several compartments • Compartments called segments, allow for increase in body size • In higher animals, segments are specialized and more complex

  10. Trends in Invertebrate Evolution • Many organisms also have symmetry • Radial symmetry = body parts repeated around an imaginary line drawn through center of the body • Bilateral symmetry = left and right sides are identical

  11. Trends in Invertebrate Evolution • Organisms with bilateral symmetry also show cephalization, which means that they have a distinct front end and back end (anterior and posterior)

  12. Invertebrate Phyla • Phylum Porifera (sponges) • Phylum Cnidaria (cnidarians) • Phylum Platyhelminthes (flatworms) • Phylum Nematoda (nematodes or roundworms) • Phylum Mollusca (mollusks) • Phylum Annelida (annelids) • Phylum Echinodermata (echinoderms) • Phylum Arthropoda (arthropods) • Phylum Chordata (invertebrate chordates)

  13. Phylum Porifera (sponges) • Simplest multicellular animals • Asymmetric • Mostly independent, but can live together • Mostly marine, some in freshwater

  14. Phylum Cnidaria (cnidarians) • Jellyfish, corals, sea anemones, hydras • Mostly marine, few freshwater • Radial symmetry

  15. Phylum Plathyhelminthes (flatworms) • Bilateral symmetry • Show cephalization • Mostly parasitic

  16. Phylum Nematoda (nematodes AKA roundworms) • First to show a tube-like digestive system (mouth at one end, anus at the other end) • Some are microscopic, some can be a meter long

  17. Phylum Mollusca (mollusks) • Clams, snails, squids • Can live everywhere, even on land • Many have outer shells for protection • Sea snails secrete chemicals to avoid predators • Clams, mussels, oysters, scallops are bivalves (double shells) and can shut when in danger • Squids and octopuses also emit dark ink to confuse predators

  18. Phylum Annelida (annelids) • AKA segmented worms • Many are detritus feeders • Live on land and in water

  19. Phylum Echinodermata (echinoderms) • Star fish, sea cucumbers, sea urchins) • Live entirely in water • Spiny skin • Have five-part radial symmetry

  20. Phylum Arthropoda (arthropods) • Most successful animals - largest and most diverse phylum • Jointed-leg animals • Exoskeletons • Divided into three groups

  21. Phylum Arthropoda (arthropods) • Subphylum Chelicerata includes spiders and scorpions • Named by mouth parts (chelicerae) • Mostly carnivorous, can be posionous • Class Crustacea includes lobster, shrimp, crab • Class Insecta contains ½ of all arthropods

  22. Phylum Chordata • Invertebrate chordates • Link between invertebrates and vertebrates • Endoskeleton with notochord, to which muscles are attached • Ex: Sea squirt

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