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Chapter 9 Multicellular and Tissue Levels of Organization

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1 Chapter 9 Multicellular and Tissue Levels of Organization
Phylum Porifera – The Sponges Zoology

2 Origins of Multicellularity
Why become multicellular? Division of labor, specialized cells with specific functions. Two Hypotheses: Colonial Hypothesis – a dividing protist remained together. Syncytial Hypothesis – formation of plasma membranes in a protist may have produced a small, multicellular organism.

3 The common ancestor of living animals
May have lived 1.2 billion–800 million years ago May have resembled modern choanoflagellates, protists that are the closest living relatives of animals. Identical to a type of sponge cell – choanocytes – used in feeding. Single cell Stalk

4 Characteristics of Porifera
Porifera – means “pore bearing” 9,000 + species, mostly marine Asymmetrical Body Plan Three Cell Types: pinococytes, mesenchyme cells, and choanocytes Water canal system Cellular Organization, but no tissue or organs Images courtesy and copyright Dr. John Hooper, Queensland Museum

5 Porifera Diversity – Three Classes
Class Demospongiae Class Hexactinellida, Staurocalyptus sp. Class Calcarea – composed of calcite Images Courtesy:

6 More than Just Cute! Have a division of labor
Pinacocytes – thin, flat cells that line outer surface, may contract and change shape of sponge or regulate water entry – porocyte. Mesohyl – jellylike layer below pinacotye layer. Mesenchyme Cells – ameboid cells moving about in mesohyl; for reproduction, secreting structures, food transport and storage. Choanocytes – flagellated cells below mesohyl that line inner chamber(s); create water current and filter microscopic food.




10 Other species make a fibrous protein of collagen - spongin
Some species make spicules – thorn-like projections that provide structural support and protection; made of calcium carbonate by ameboid cells. Other species make a fibrous protein of collagen - spongin Spicules

11 Sponges (Porifera)

12 Water Currents for Everything!
Choanocytes use their flagella to create water currents through external pores called – ostia (sing. ostium,); incurrent pores. Bring food (bacteria, protists, etc.) and oxygen and remove metabolic wastes from the center of the sponge – spongocoel. Choanocytes use collar-like rings to filter food. Wastes and water flow out a central osculum (plural, oscula); an excurrent pore.

13 Sponges (Porifera)

14 Sponge Anatomy Choanocyte

15 1. Ascon Body Form Simplest canal system with a central spongocoel lined with choanocytes and with many ostia opening directly into spongocoel Least common

16 2. Sycon Body Form Sponge wall is folded Water enters via dermal pores
Canal system with a central spongocoel into which many radial canals empty. Choanocytes line radial canals

17 3. Leucon Body Form Branched incurrent canals lead to choanocyte-lined chambers No spongocoel Increased surface area = larger volume of water movement Most common

18 Let’s Eat! Choanocytes filter microscopic food and trap in collar.
Placed into food vacuole and digested by lysosomes. Digested food is passed to amoeboid cells for transport to other cells – beginnings of specialization.

19 The Importance of Water Currents
Respiration (gas exchange), Metabolism, and Excretion all done by direct diffusion with water. No nervous system – no responsiveness. Defenses – may produce some irritating chemicals if touched; chemical defense against predators, fish, sea stars, etc.

20 Reproduction Sponges are monoecious (both sexes in the same individual – hermaphrodite). Do not self-fertilize. Why? Choanocytes become sperm. Other choanocytes and amoeboid cells become eggs. Released from oscula and exteranl fertilization. Larvae are free-swimming.

21 Free-swimming larvae settle to the bottom and ….
…become sessile (attached to the bottom) adults

22 Alternatives to Sex Asexual reproduction from internal, resistant capsules – gemmules. Gemmules – are masses of ameboid cells that are released when parent dies. Dormant stage - resistant to freezing and drying. Pieces broken off can become a new sponge – fragmentation. Grow new pieces – budding.

23 And their good to eat, too!
Sponge Cake, Anyone?

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