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What is an Animal?
Characteristics of Animals
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Animals are eukaryotic, multicellular organisms
Cells do not have cell walls
Can move in some way
All animals are heterotrophic: must consume
food (autotrophs like plants or other
heterotrophs like hamburger from a cow) to
obtain energy.
• Digest food
• Have specialized cells
Development
• Animals share similar developmental stages.
– Fertilization: Sperm (1N half the genetic material of
the male adult) joins with the egg (1N half the genetic
material of the female adult) The fertilized egg is
called a zygote.
– Cell division: The zygote divides through the process
of cleavage. Once the cell division starts the name
changes from zygote to embryo.
– The cells continue to divide until they form a hollow
ball of cells called the blastula. The hollow space is
called the blastocoel.
– The cells on one side of the blastula buckle inward to
form the gastrula.
Development continued
The layer of cells on the outer surface of the
gastrula make the ectoderm. This continues to
grow and eventually becomes the skin and
nervous system.
• The layer that folds in on the gastrula is called
the endoderm. The endoderm continues to grow
and the cells eventually differentiate into the
digestive system and digestive organs.
• Some animals develop an additional layer
between the ecto and endoderm. The cells
come from the endoderm and the new middle
layer is called the mesoderm. The mesoderm
becomes the muscular, respiratory, circulatory,
and excretory systems.
Development continued
• From the gastrula on the cells continue to grow
and differentiate into tissues, organs, and
systems.
• If the opening (blastopore) formed by the
invagination (buckling) of the gastrula becomes
the mouth the animal is called a Protostome. EX.
Earthworms, insects
• If the mouth of the animal develops from another
area (not the opening, or blastopore) on the
gastrula it is called a Deuterostome.
Image taken from http://www.lander.edu/rsfox/310asteriasLab_files/image010.jpg
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