Introduction to Metazoa

Metazoans

Multicellular organisms

Animals in kingdom Animalia

Believe to have evolved from protozoans; specially choanoflagellates

Metazoan Chracteristics

• Eukaryotic Heterotrophs

• Polarized along an anterior-posterior locomotory axis

• Most motile

Ground Plan

• Specialized cells organized as tissues

• Primary Tissues: epithelial & connective

• Epithelium: sheets of cells bound to each other by cell-adhesion molecules.

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)

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.

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.

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.

Direct Development

• 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.

Reproductive Adaptations

• 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.

Maternal Provisioning

Oviparous

Viviparous

• 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

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.

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.

DIFFUSION

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.

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.

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.

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

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.

Peramorphosis

• 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.

Origins of Metazoa

• 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.

Sources• http://www.google.com.ph/imgres?q=metazoa&hl=fil&biw=1366&bih=585&gbv=2&tbm=isch&tbnid=ITnAcb6yU3INiM:&imgrefurl=http://

www.ucmp.berkeley.edu/phyla/phyla.html&docid=nVJrWUumXbrhFM&imgurl=http://www.ucmp.berkeley.edu/phyla/animcoll.jpg&w=405&h=342&ei=feKwT_6kOOXjmAXEjsmlCQ&zoom=1&iact=rc&dur=169&sig=114980428662261257468&page=1&tbnh=122&tbnw=144&start=0&ndsp=23&ved=1t:429,r:0,s:0,i:67&tx=100&ty=36

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