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Traits:
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Cells have DNA
Cells have a nucleus in them
Cells have mitochondria
Multicellular
Cells are held together with collagen.
Sexual reproduction
Early embryo forms as a hollow ball of cells called a blastula
This blastula opens into a tube “tail end first”
Has a head and is bilaterally symmetrical (you can draw one
line down the middle)
Cells organize themselves into true tissues (muscle, nerves)
Cells organize themselves into true organs (heart, liver,
stomach)
Has a hollow space between the gut and outer body wall
where organs are (this space is called a coelem—
pronounced “see-lome”
Segmented body
Internal Skeleton
Backbone, Spinal chord and braincase,
Bony armored plates in skin
Teeth are bone (not true enamel teeth)
Internal skeleton is made of cartilage, not bone (cartilaginous)
No jaw
Many small gill openings on either side of the body
No paired fins
A1:2 Sacabambaspis janvieri Early Odovician
Sacabambaspis was a tadpole shaped fish that lived 450 million years ago. It swam by waggling its tail, but had no fins, which would have made braking and
steering very difficult. Two tiny headlightlike eyes.
(Adapted from Dixon, p.70)