What do echinoderms have instead of brains?

Instead of a brain, echinoderms have a ring of nerves located around their mouth area that governs their nervous responses. This ring coordinates their motion, their eating, basically anything that requires nerve control.

Explanation:

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during the process of dna replication, brdu can replace thymidine and incorporate into the newly synthesized dna, cell proliferation thus can be reflected in the extent of brdu incorporation.