BIOL 111 Lecture Notes - Turbellaria, Triploblasty, Schistosomiasis
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5 Apr 2012
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11/10/11
Embryonic development:
Changes that occur between zygote stage and the multicellular organism.
Zygote: diploid, resulting from union of two haploid gametes, 1st cell of 2nd gen
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