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Extends far beyond humans and otheranimals we may encounter
Figure 32.1
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Animal are multicellular, heterotrophiceukaryotes with tissues that develop fromembryonic layers
Several characteristics of animals Sufficiently define the group
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Animals are multicellular eukaryotes
Their cells lack cell walls
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Most animals reproduce sexually
With the diploid stage usually dominating thelife cycle
After a sperm fertilizes an egg
The zygote undergoes cleavage, leading to
the formation of a blastula The blastula undergoes gastrulation
Resulting in the formation of embryonictissue layers and a gastrula
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Cleavage Cleavage
Blastocoel
BlastocoelEndoderm
Ectoderm
Blastopore
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Early members of the animal fossil record
Include the Ediacaran fauna
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The Cambrian explosion
Marks the earliest fossilappearance of many major groupsof living animals
Is described by several currenthypotheses
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Animals can be categorized According to the symmetry of their bodies, or
lack of it
Some animals have radial symmetry Like in a flower pot
Radial symmetry. The parts of a radial animal, such as a sea anemone (phylumCnidaria), radiate from the center. Any imaginary slice through the central axis divides theanimal into mirror images.
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A bilateralanimal, such as a lobster (phylum Arthropoda), has a left side and aright side. Only one imaginary cut divides the animal into mirror-image halves.
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Based on certain features seen in earlydevelopment
Many animals can be categorized as havingone of two developmental modes:protostome development or deuterostomedevelopment
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In protostome development Cleavage is spiral and determinateIn deuterostome development Cleavage is radial and indeterminate
Eight-cell stage Eight-cell stage
Spiral and determinate Radial and indeterminate
In general, protostomedevelopment begins with spiral,determinate cleavage.
Deuterostome development ischaracterized by radial,indeterminate cleavage.
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In protostome development The splitting of the initially solid masses of
mesoderm to form the coelomic cavity iscalled schizocoelous development
In deuterostome development Formation of the body cavity is described asenterocoelous development
Archenteron
Blastopore MesodermCoelom
BlastoporeMesoderm
Schizocoelous: solidmasses of mesodermsplit and form coelom
Enterocoelous:folds of archenteronform coelom
Coelom Coelomformation begins in the gastrulastage. In protostome
development, the coelom formsfrom splits in the mesoderm(schizocoelous development). Indeuterostome development, thecoelom forms from mesodermaloutpocketings of the archenteron(enterocoelous development).
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In protostome development The blastopore becomes the mouthIn deuterostome development The blastopore becomes the anus
Anus
Anus
Mouth
Mouth
Mouth developsfrom blastopore
Anus developsfrom blastopore
Digestive tube
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