What happened to the head and nervous system of bivalves? With what could such modifications be connected?

Bivalve molluscs are extremely sedentary with rare exceptions. Usually they burrow into the ground, and by the way they are fed, they are referred to as filter feeders. Under such conditions, they do not need well-developed sense organs, because they do not need to move around to search for food on their own. However, they have statocysts (pebbles that help determine position in space), integumentary receptors and organs of chemical sense, some have primitive eyes that pick up light fluctuations.

In this regard, these mollusks have a reduced head, and the nervous system is poorly developed. It consists of three paired ganglia connected to each other. One pair of ganglia are located near the esophagus, in the leg, and also under the posterior guard muscle.