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.

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.

13. Direct Development

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.

15. Reproductive Adaptations

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.

35. Origins of Metazoa

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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