The creatures of the univalve variety, which are also known as gastropods, have a single shell that is usually coiled. They have a distinct head with tentacles and a rasping tongue (radula). Most of the 105,000 known species in this group have shells. Half of the species are marine while the remainder is either terrestrial or fresh water. The sexes are separate in most marine species. Free-swimming larva emerges from eggs laid into the water. This larva eventually grows into a shell adult. The average life expectancy of a gastropod is five years while some have survived to 30 years. They may be carnivorous, herbivorous or parasitic.