Presentation on theme: "Chapter 9 Multicellular and Tissue Levels of Organization Phylum Porifera – The Sponges Zoology."— Presentation transcript:
Chapter 9 Multicellular and Tissue Levels of Organization Phylum Porifera – The Sponges Zoology
Origins of Multicellularity Why become multicellular? Division of labor, specialized cells with specific functions. Two Hypotheses: 1.Colonial Hypothesis – a dividing protist remained together. 2.Syncytial Hypothesis – formation of plasma membranes in a protist may have produced a small, multicellular organism.
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
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
Porifera Diversity – Three Classes Class Calcarea – composed of calcite Class Hexactinellida, Staurocalyptus sp. Staurocalyptus Images Courtesy:http://www.palaeontologie.geo.lmu.de/molpal/calcarea_introduction.html Class Demospongiae
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.
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
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.
Sponge Anatomy Choanocyte
1. Ascon Body Form Simplest canal system with a central spongocoel lined with choanocytes and with many ostia opening directly into spongocoel Least common
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
3. Leucon Body Form Branched incurrent canals lead to choanocyte- lined chambers No spongocoel Increased surface area = larger volume of water movement Most common
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.
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.
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.
Free-swimming larvae settle to the bottom and …. …become sessile (attached to the bottom) adults
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.
And their good to eat, too! Sponge Cake, Anyone?