Introductionto metazoa 2012
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Metazoans are multicellular eukaryotic organisms classified as animals in the kingdom Animalia. They are believed to have evolved from protozoans like choanoflagellates. Key characteristics include being polarized along an anterior-posterior axis, having specialized cells organized into tissues, and undergoing complex development from a zygote to a multicellular embryo. Larger body size in metazoans allows for cell specialization but requires circulatory systems and other adaptations for nutrient/waste exchange.
Introductionto metazoa 2012
- 2. Metazoans Multicellular organisms Animals in kingdom Animalia Believe to have evolved from protozoans; specially choanoflagellates
- 3. Metazoan Chracteristics • Eukaryotic Heterotrophs • Polarized along an anterior-posterior locomotory axis • Most motile
- 4. Ground Plan • Specialized cells organized as tissues • Primary Tissues: epithelial & connective • Epithelium: sheets of cells bound to each other by cell-adhesion molecules.
- 6. Ontogeny • “Origin of being” • Development of an organism • Zygote formed by the union of egg and sperm nuclei • Multicellular embryo is formed in process known as cleavage (division)
- 8. Embryo Development • Blastula: 1 cell layer thick hollow ball or solid ball of cells • Gastrulation: invagination of one wall of blastula -> 2 layered gastrula • Morphogenesis • Ectoderm & endoderm form around gelatinous blastocoel.
- 10. Cells, Tissues, Skeletons • Protozoans posses very little specialization • Most protozoans rely on organelles to carry out all functions. • Different tissue types allow for a partioning of labor • Damage cells can be regenerated; however if a paramecium is damaged the whole oraganism dies.
- 12. Indirect Development • Life cycle includes a larval stage • Larva: independent stage with different anatomy & niche. • Biphasic life cycle: benthic adult & planktonic larva. • Larva settles and undergoes metamorphosis into an immature jevenile.
- 14. • Embryo develops directly into jeveniles without a larval stage • Considered to be a derived trait • Indirect development with external fertilization & planktonic larva is the ancestral pattern.
- 16. • Improve chances of fertilization & embryo survival: increase synchrony & proximity • Hermaphroditism: common in species with small population densities & sessile lifestyles. • Any nearby individual is a potential mate • Most hermaphrodites cross-fertilize than self fertilize.
- 17. Maternal Provisioning Oviparous Viviparous
- 18. • Adaptations providing physical protection & nutrients to offspring are very valuable. • Oviparous: eggs spawned before or just after fertilization • Viviparous: internal fertilization, embryos, gestate in maternal body & release larva or juveniles. • Brooding: eggs released from mother, but are retained on or taken back to her body
- 19. Functional Consequences of Body Size • Most metazoans are 0.5 mm – 1m in size • Prokaryote (seed) Protozoan (grapefruit) Animal (stadium) • Cell specialization improves efficiency • Requires functional compartmentalization & cellular integration.
- 20. Size, Surface Area and Volume • SA:V is significantly affected by increases in body size. • As a cell grows larger, its area is squared & its volume is cubed. • SA (supply) will not be able to support cell volume (demand) • Limits exchange of gases, nutrients and wastes.
- 22. DIFFUSION
- 23. Size and Transport • Rates of diffusion slow drastically over great distances. • Effective diffusion distance is roughly 0.5mm for most animals. • Body diameters larger than 1mm may be diffusion-limited. • Circulatory system needed for bodies larger than 1mm in diameter.
- 25. Size and Metabolism Metabolic rate increases with body size. Poikilotherms (cold-blooded animals) consume 8 times more mass-specific energy than protozoans. Homeotherms (mammals & birds) cosume 29 times more energy than a poikilotherm of equal term.
- 27. Advantages of a Large Body Size • Mass specific decreases in metabolic rate • Reduced risk of predation by protozoa • Larger metazoans can prey upon protozoans • Motile metazoans move faster than protozoans • Multicellularity allows ability to regenerate.
- 28. Ontogeny & Phylogeny • Metazoan ontology includes developmental stages subject to natural selection • Heterochrony: changes in the timing of developmental events- allows potential for evolutionary change. • Two types of heterochrony • Pedomorphosis & Peramorphosis
- 30. Pedomorphosis • A trait of descendent species resembles an ancestral larval or jevenile developmental trait • Results in smaller and simpler descendents with short generation times. • Common in species living in unpredictable or changing enviroments • Adapted to colonize entirely new habitats.
- 33. Peramorphosis
- 34. • A trait of descendant species that develops beyond the ancestral trait • Results in larger & more complex descendants with longer generation times. • Favored in constant or predicatable enviroments. • Larger body size is a major trend in metazoan evolution.
- 36. • Colonial Theory: Metazoans are derived from colonial flagellated protozoans. • Choanoflagellates & metazoa are sister taxa • Spherical colony of flagellated cells divided by mitosis, but daughter cells held within ECM.
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