Animals in the same phylum may have different nervous system structures. for example, octopuses have large, complex brains while clams have a simple network. ladder-like ring-like nerve net

Ring-like.

Explanation:

The nervous system of the mollusks consists, basically, of a ring of nerves around the esophagus and a series of nerve cords that follow to the other regions of the body.  The sensory system is very varied and depends on the group. Some have structures, called statocysts, capable of perceiving environmental variations, others have sensory tentacles and eyes.