Some species of jelly fish have good sight: they have 24 eyes, can sense light and form images of their surroundings. Little is known, however, about what they actually see. They don't need to see to mate. Females and males just get together and fertilize eggs in a mass spawning, though some species do appear to mate one-on-one.
The most dangerous jelly fish, in Australia, is the Box Jellyfish (sea wasp or stinger). Some can immobilize while others can kill. The Portuguese man of war is held to be very dangerous, but despite its name it cannot kill humans. Chironex flecken, an Australian box jellyfish, can be lethal. A similar type, Chironex yamaguchii, has killed people in Japan.
Evolution of jellyfish has been pinned down by DNA extracted from tissue samples. It is now known how species are related. It was found that several types cause Irukandji Syndrome which leaves sufferers with body pain, severe depression and feelings of impending doom. If you are stung by members of this group then these symptoms can be expected. Some types have been isolated from others for a long period of time. Sea level changes from tectonic plate movements apparently create new species.