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Evolution of animals

All animals are multicellular and heterotrophic

They ingest whole foods and digest internally

Lack a cell wall

Are mobile and have muscles and nerves

Reproduce sexually and have a diploid lifecycle

Monophyletic (Both vertebrates and invertebrates)


The colonial flagellate hypothesis

Animals are descended from an ancestor resembling a hollow spherical colony of flagellated cells

Individual cells within the colony become specialized

Two tissue layers could have arisen by infolding of certain cells into a hollow sphere

Implies that radial symmetry proceeded bilateral symmetry


Evolution of body plans

All of the diversified body plans characteristic of animals today were present by the Cambrian period

Slight shift in the DNA code an expression of certain genes sponsorable for the differences that arise during development.



Asymmetric- no particular body shape
Radial symmetry- body organized circularly
Bilateral symmetry- definite right and left halves


Embryonic development

The first tissue layer that appears in embryonic development is called germ layers

They give rise to organs or organ systems

Diploblastic- two tissue layers ectoderm and endoderm found in Cnidarians

Triploblastic- three tissue layers ectoderm mesoderm and endoderm

They are either protostomes or deuterostomes
- Protostomes first embryonic opening becomes the mouth
Deuterostomes. Second embryonic opening becomes the mouth


Protostomes vs deuterostomes

Cleavage- cell division immediately after fertilization

Protostomes- spiral cleavage determinate fate( will die if separated)
Deuterosomes- radial cleavage indeterminate fate

Protostomes sit in valley of original cells deuterostomes stack up on top of the original cells


Three basic kinds of body plans for triploblastic animals

Acoelomates- no body cavity
Pseudocoelomates- body cavity between mesoderm and endoderm called pseudocoelom
Coelomates- body cavity entirely within the mesoderm called the coelom.