Evolutionarily, as animals increased in complexity, their nervous tissue began to cluster together. in active animals, the tissue clustered toward the head. this cluster became the and the tendency to cluster toward the head is referred to as

brain, cephalization

Explanation:

Cephalization is an evolutionary mechanism which led to the concentration of nerve ganglia, sense organs and mouth at animal's front end giving rise to a head region. Clustering of nerve ganglia in head produced the brain region.

It is a trend adopted in more advanced organisms due to movement and bilateral symmetry. Three groups of animals show cephalization: arthropods (insects), cephalopod molluscs (octopus, squid) and vertebrates (mammals, birds, fishes). Cephalization has been studied extensively in vertebrates. As the groups advance in vertebrates, the brain becomes more elaborate and complex.

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well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."