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Phylum Cnidaria Phylum Cnidaria • jellyfish, hydras, sea anemones, corals • carnivorous • radial symmetry • have stinging cells called cnidocytes which contain nematocysts • nematocysts are poison filled harpoons used for catching prey Body Symmetry Radial Symmetry Bilateral Symmetry Posterior end Dorsal side Anterior end Ventral side Plane of symmetry Planes of symmetry Cnidarian Body Plan • radial symmetry with tentacles • two stages to life cycle – Polyp – Medusa • body surrounds an internal space, gastrovascular cavity • gastrovascular cavity lined with gastroderm (tissue layer) • epidermis is outermost layer • mesoglea between gastroderm and epidermis (jelly-like) Polyp • Sessile • cylindrical body • mouth faces upwards • tentacles point up Medusa • Motile • bell-shaped body • mouth underneath • tentacles point down The Polyp and Medusa Stages Epidermis Mesoglea Gastroderm Tentacles Mesoglea Gastrovascular cavity Mouth/anus Mouth/anus Gastrovascular cavity Tentacles Medusa Polyp Feeding • prey is paralyzed and pulled into the gastrovascular cavity • digestion is extracellular • nutrients are absorbed by gastroderm • nutrients dispersed by diffusion • anything not digested leaves through the mouth/anus Respiration, Circulation, Excretion • all are done by diffusion through the body wall Response • specialized cells of the nerve net receive stimuli • nerve net is evenly distributed throughout the body • no true brain • also have cells to determine light intensity and gravity Movement • done by hydrostatic skeleton in polyps • done by jet propulsion in medusa (like a water pump) Reproduction: Sexual • • • • • fertilization is external separate male and female organisms zygote forms a free-swimming larva larva grows into a new polyp polyp then buds to form new medusa Reproduction: Asexual • polyps reproduce by budding • new polyps may grow off the side of existing polyps • polyp may produce medusas Jellyfish Life Cycle Female medusa (2N) Adult medusas reproduce sexually by releasing gametes into the water. MEIOSIS Fertilization occurs in the open water, producing many diploid zygotes. Egg (N) FERTILIZATION Sperm (N) Zygote (2N) Male medusa (2N) Young medusa The polyp buds to release young medusas. Haploid Diploid Each zygote grows into a ciliated larva. The larva eventually attaches to a hard surface and develops into a polyp. Swimming larva Budding polyp Polyp Jellyfish • class Scyphozoa (cup animal) • life is spent mostly in medusa stage • larva and polyp stage are usually the same • reproduce sexually Hydras • class Hydrozoa • usually grow in colonies • no medusa stage in life cycle of freshwater hydras • may reproduce either sexually or asexually Anemones and Corals • class Anthozoa (flower animals) • only have a polyp stage in their life cycle • many are colonial • reproduce sexually (some asexually) • larva forms a new polyp
