Most members have a shell, which is in one piece and coiled or spiralled; it usually opens on the right hand side (viewed with the shell apex pointing up). Some species have an operculum, a lid or trapdoor to close the shell. In some, the slugs, the shell is absent, and the body is streamlined. The best-known gastropods are land slugs and snails, but more than half of all species live in a marine environment. Marine gastropods include herbivores, detritus feeders, carnivores and a few ciliary feeders, in which the radula is reduced or absent. The radula is usually adapted to the food that a species eats.