Mollusks (including clams) have a central nervous system, even a primitive brain: "In the nervous system typical of mollusks, a pair of cerebral ganglia (masses of nerve cell bodies) innervate the head, mouth, and associated sense organs. From the dorsal cerebral ganglia, two pairs of longitudinal nerve cords arise..." http://www.britannica.com/eb/article-35785/mollusk After all, it has complex behaviors (feeding, mating, defense, etc) that need to be coordinated. All animals have behaviors in response to external stimuli signaled by sense organs. But "feelings" is a different story. Complex feelings that are obviously human in nature (embarrassment, jealousy, guilt, etc) are mediated by our cerebral cortex -- the "thinking" part of our brain that is highly developed in humans alone. Pain is a particularly interesting "feeling." All animals have avoidance behaviors for potentially harmful stimuli, but the unpleasant emotional component that goes along with people's perception of pain is modulated by our cerebral cortex and limbic system (among many pathways), none of which can be found in a mollusk, arthropod, or other invertebrate "lower animal." There is no evidence that when you step on an ant, the ant "hurts" before it dies. In fact there is much debate about whether a fish, a vertebrate like us with a similar nervous system, has a big enough brain to experience pain from a fishhook stuck in its mouth. All mammals, by contrast, clearly react to painful stimuli in ways more closely resembling humans. Mammals appear to "suffer" pain. The other vertebrates (amphibians, reptiles, and birds) are probably somewhere in between. If your moral/ethical values regarding the treatment of animals is based on the humanistic concept of suffering, then be kind to all mammals, think twice before hooking a fish, but don't worry about clams.