Why are there two types of endoplasmic reticulum?

Hi there,

The two types of endoplasmic reticulum is the rough endoplasmic reticulum (rough ER) and smooth endoplasmic reticulum (smooth ER)

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the cladogram shown here shows similarities and differences among several classes of vertebrates. anatomically, the four have limb structure as their common ancestor. we might separate the mammals from the other three classes because they a) divergent; do not lay eggs b) identical; are warm-blooded c) varied; a four chambered heart d) homologous; have mammary glands