Some mollusks, like the octopi, have large, complex brains, and are very intelligent animals. The digestive system is very similar to that of other complex invertebrates, and all other complex, multi-celled animals, for that manner. The basic components are the mouth, coelom, and anus. Reproduction in mollusks is as diverse as the group is. The common reproductive charateristic of the group is egg-laying, however.
Some gastropods are hermaphroditic, that is, they have both male and female parts.
All animals, including fish, have senses that allow them to distinguish themselves from other species. For fish, sight is probably a very important cue, but bony fish have an additional sense that allows them to detect vibrations in the water. This sensory system is located along the sides of the fish body, called the lateral line. It's a series of pits along the side of the fish. Scientists don't know all of the senses theat this line can detect, but we know it senses vibrations in the water, and may be an important sense for allowing fish to recognize the vibrations of a fish of their own species, allowing them to swim in synchrony in tightly-packed schools.