The family Diadematidae has around 30 extant species, including some of the most common tropical sea urchins species (Diadema antillarum, Diadema setosum, Astropyga radiata...).
These are big regular sea urchins, with hollow and usually very long spines, some of them being venomous (but not dangerous for humains). Some are bright colored (especially red or blue), but others are dark, sometimes with ornamental patterns called iridophores (especially Astropyga and Diadema).
One of their most obvious characteristic is to have an "anal papilla" (or "anal cone") on the top of the test : it can be very obvious in some species (Echinothrix calamaris, Diadema setosum) or nearly hidden (Echinothrix diadema, Centrostephanus longispinus).
Most of them inhabit tropical waters of the indo-pacific.
Among their skelettal characteristics, they have trigeminate ambulacra, with perforated tubercles, almost always crenulate.
Like other sea urchins diadematids are sensitive to touch, light, and chemicals; additionally they do have eyes (eye spots) which is in contrast to other sea urchins. Because of this they can follow a threat with their spines.
Media related to Diadematidae at Wikimedia Commons