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Some of the first animals to develop an endoskeleton were _____.

mollusks,
arthropods,
annelids,
echinoderms

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echinoderms

Endoskeleton is a type of skeletal structure definitive of its formation according to specific organism structure. Endo meaning inside particularly elucidates that the skeletal structure of a living being is internal or inside its body. Some of the early organisms were echinoderms and chordates.
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