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Platyhelminthes and Cnidaria

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McGill University
Biology (Sci)
BIOL 111
Irene Gregory- Eaves

11/10/11 Embryonic development: Changes that occur between zygote stage and the multicellular organism. Zygote: diploid, resulting from union of two haploid gametes, 1 cell of 2 gennd Embryo: young animal or plant while still contained within protective structure 2 key processes: 1. cleavage = cell division – forming of many cells 2. gastrulation = in-folding – from of 2-3 embryonic tissue layers Zygote  cleavage  morula (solid ball)  blastula (hollow)  gastrula (in- folding) Gastrula: layers of embryonic cells = germ layers - exterior: ectoderm = epidermis/nervous tissue - interior: endoderm – lining of digestive/respiratory systems - some only have 2 layers = diploblastic - third layer in centre: mesoderm – internal systems, muscles, hear, stomach (triploblatic) - archenteron = cavity formed when envagination occurs = gut cavity - blastopore = mouth/anus Tissues: enable a more cohesive organization of the body plan and complex movement Gut: enable extracellular digestion (large food particles) Tissues evolve from embryonic tissues (tree splits at sponges) Types of symmetry: 1. Asymmetrical: no axis divides body into equal halves, never symmetrical no matter how you slice it (ex. sponges) 2. Radial symmetry: cut along one axis in the centre of the body on many planes/angles and you’ll find equal halves (ex. Cnidarians) 3. Bilateral symmetry: single axis that divides the body into two equal halves, typically have a head (between eyes = equal halves – midsagittal plane) a. -yields, anterior/posterior, dorsal/ventral (ex platyhelmenthes) Star fish: larval stage/ancestors were bilaterally symmetrical Cnidarians and Platyhelminthes Cnidarians: - radial symmetry, blind gut, diploblastic - jellyfish, sea anemones, hydrozoans - mostly marine (11000 organisms) - microscopic to meter in length - carnivorous 11/10/11 - diploblastic - nervous/muscle tissues General body plan: Internally: - jelly mesoglea cavity - gastrodermis: interior of cavity arises from endodermis - exterior: ectodermis arises from ectoderm - mouth: from blastopore - gastrovasular cavity: archenterone digestion/respiration (blind gut) - diploblastic Forms: 1. Polyp: sea anemone: stay in one plane (sessile) 2. Medusa: jellyfish (motile), muscle that surround gastrovascular cavity that forces water out that pushes jellyfish up Energy: - carnivorous  use cnidocyts to capture pray: contain organelle “nematocysts” = Have spines/barbs/venom - extracellular digestion in gastrovascular cavity - corals can obtain a large proportion of their energy from symbiotic algae Nervous system: Have nerve net with little to no integration of nerve signals, direct line between sensors cells and muscles (effectors) Reproduction: - Sexual and asexual Ex. Polyp, medusa formed asexually have sexes, release egg/sperm into water which make planula larva which settle and make polyp which can reproduce by budding Cnidarians diversity 1. Hydrozoa - Polyp dominant Ex. hydra: fresh water, no medusa, moves by gliding/floating i. Produce gas bubble at basal disk allow from floating in water column ii. 2-5mm Ex. Portuguese ma of war: colonial polyps and medusa specialized - tentacle contain cnidocysts w/ venom
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