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Lecture 15

BIO153 Lecture 15.pdf

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University of Toronto Mississauga
Christoph Richter

2009 BIO153: Lecture 15 Invertebthtes (I) Mar 16 , 2009 Porifera (sponges): The simplest of the animals. ▯ sessile, mostly marine ▯ reproduce sexually and asexually ▯ cellular level of organization – some argue that they are more colonial than truly multicellular ▯ show no real body symmetry ▯ choanocytes (specialized feeding cells) look like choanoflagellates (protists) The next step: 2 tissue layers Diploblastic animals: only endoderm and ectoderm (no mesoderm) ▯ radiata (animals with radial symmetry): a paraphyletic group 2 phyla: Cnidaria and Ctenophora 1. Cnidaria: ▯ radial symmetry; 2 cell layers ▯ blind guts (mouth = anus) ▯ 2 body plans: sessile polyp and floating medusa (may alternate between polyp & medusa forms) ▯ carnivorous ▯ stinging cells (nematocysts) ▯ no “true” muscles ▯ no mesoderm! ▯ jellyfish, sea anemones, coral and hydrozoans 2. Ctenophora ▯ radial symmetry; 2 cell layers ▯ complete guts (mouth + 2 anal pores) ▯ ctenes: plates of cilia (move by cilia, not muscles) ▯ comb jellies Evolutionary trends in animals: ▯ diploblastic ▯ triploblastic ▯ radial ▯ bilateral symmetry 1 The 2 main lineages of bilateral, triploblastic animals are the Protostomes & Deuterostomes Protostomes & Deuterostomes are defined by 3 events that occur in early embryonic development: Embryonic development in animals: Zygote ▯ blastula ▯ gastrula ▯ further development Protostomes and Deuterostomes differ in: 1. type of cleavage pattern (formation of blastula) 2. fate of blastopore (during gastrulation) 3. formation of the coelom (during later development) 1. Cleavage: zygote divides to form ball of cells (blastula): Protostomes have spiral cleavage; Deuterostomes have radial cleavage ▯ 2. Gastrulation: cell movements result in organization of the embryo It is not birth, marriage, or death, but gastrulation which is truly the most important time in your life." Lewis Wolpert (1986) Gastrulation: the ball of cells formed by cleavage (the blastula) invaginates to form the gut & the embryonic tissue layers (germ layers) – also forms the blastopore (opening at one pole of the embryo) 2 The fate of the blastopore differs in protostomes & deuterostomes: In Protostomes, the blastopore becomes the mouth; in Deuterostomes, the blastopore becomes the anus. Gastrulation produces the 3 germ layers: endoderm, mesoderm and ectoderm: endoderm Lining of digestive and respiratory tracts; glands mesoderm Muscle, excretory system, gonads, circulatory system, bones & cartilage, dermis ectoderm Epidermis, brain, nervous system Fate map of Deuterostome embryo: 3. Formation of coelom (a body cavity lined in mesoderm): 3 However, not all Protostomes produce a “true” coelom: 3 conditions: a) Acoelomate: no fluid-filled cavity b) Pseudocoelomate: cavity not fully lined with mesoderm (cavity is between mesoderm and endoderm). The pseudocoel is a fully functional cavity, but is formed differently than the “true” coelom – it develops directly from the blastocoel (the fluid-filled cavity in the blastula). It is thought that the pseudocoelomate condition is a reversal (ancestors of the pseudocoelomate animals hade “true” c
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