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Lecture 11

Cnidarians & Platyhelminthes (Lecture 11)

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

Biology Lecture 11: Cnidarians & Platyhelminthes Negin Arbabi Pre-lecture Vocabulary  Embryonic: in a rudimentary stage with potential for further development  Zygote: the cell created by the union of two gametes, in which the gamete nuclei are also fused. The earliest stage of the diploid generation.  Embryo: a young animal while it is still confined within a protective structure such as an egg or uterus  Symbiosis: the living together of two or more species in a prolonged and intimate relationship  Parasitism: a parasite infestation, living as a parasite NOTE: most animals undergo unique embryonic development patterns and have tissues Embryonic Development changes that occur between zygote stage in the embryo stage  Zygote (a multicellular organism): diploid cell resulting from the union of two haploid st nd gametes; 1 cell of 2 generation  Embryo: young animal or plant while it is still contained within a protective structure  Two major processes occurring during embryonic development include: 1. Cleavage: cell division - Process forming many cells from one cell 2. Gastrulation: in-folding - process forming embryonic tissue layers (2-3) from one layer of cells Gastrula  Layers of embryonic cells  Germ layers: - Ectoderm: epidermis, nervous system/tissue outer layer - Endoderm: lining of digestive and respiratory system internal cell layer  Diploblastic: organisms with 2 germ layers - Mesoderm: Most internal organs: muscles, skeletal system, heart, stomach middle layer  Triploblastic: organisms with 3 germ layers - Archenteron: digestive space (gut cavity) cavity formed when invagination occurs - Blastopore: mouth or anus (or both) Functions  Tissues: enable cohesant organization of the body plan complex body movement organization  Gut: enables extracellular digestion Type of Symmetry 1. asymmetrical - no axis divides body into equal halves - ex: some sponges 2. radially symmetrical—looking down and going down the center of the body - along one axis, as long as one cuts through center, body can be divided into equal halves from many angles - ex: cnidarians - ex: starfish: pentaradially symmetrical 3. bilaterally symmetrical - single axis divides body into equal halves - midsagittal plane (between eyes) - ex: Platyhelminthes NOTE: have to specify if it’s interior or exterior Phylum Cnidaria  Jellyfish, sea anemones, corals, hydrozoans  ~11,000 species - mostly marine - microscopic to many meters - carnivorous  Diploblastic  Nervous and muscular tissue Generalized Body Plan mesoglea: a jelly, non-living layer between gastrodermis and endodermis that makes up hydrostatic skeleton gastrodermis: on the interior, arises from the endoderm epidermis: arises from ectoderm mouth: arises from blastopore—functions as anus gastrovascular cavity: digestion and respiration through here, arises from archenteron  Cnidarian body forms: 1. Polyp—sessile 2. Medusa “upside down polyp”—motile  Cnidarians obtaining energy: “cnido=nettles” - carnivorous, using cnidocysts to capture prey, inject it with venom and stick to it—contains an organelle called nematocyst - extracellular digestion in gastrovascular cavity - corals can obtain a large proportion of their energy from symbiotic algae  Cnidarians simple nervous system: - nerve net of little to no integration or processing of signals - provide direct lines of communication between sensor cells and effector cells (muscles) Sexual Reproduction they can be polyps and medusa at different stages of life Cnidarian Diversity  Class Hydrozoa (e.g. Hydra) - polyp dominant phase, most species alternate between polyp and medusa  Class Scyphozoa (e.g. jellyfish) - m
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