Members of the phylum Echiura are marine, worm-like animals found burrowing in mud and sand, under rocks and in crevices, both on the shore and sublittorally. The body is divided into two parts, an anterior proboscis with a longitudinal ventral groove and a posterior, cylindrical or sac-like trunk, which has two large, ventral setae near its anterior end. The mouth is at the posterior end of the proboscis. The species vary in length from a few millimetres to a metre or more in those species in which the proboscis is long and highly extensible. Echiurans feed on detritus. In most, the animal remains in the burrow and the proboscis is extended over the surface of the sediment and detritus trapped in mucus secreted by the proboscis. The mucous string is carried along the ventral groove to the mouth. In some species, a mucous net is secreted by the proboscis at the entrance to the burrow and the animal filter feeds. The sexes are separate and fertilization is usually external; there is a free-swimming trocophore larva.