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General introduction

The sipunculids are a small but distinct group of vermiform animals, sometimes called peanut worms. Formerly, these animals were placed in the Phylum Gephyrea, a group of uncertain affinities comprising the Classes Sipunculoidea, Echiuroidea and Priapuloidea. The unsegmented body is composed of two main parts, a broadly cylindrical trunk and, anteriorly, a more slender, extensile introvert. They occur in all seas and certain species are known to be widely distributed from worldwide areas. They are exclusively marine and do not penetrate into estuaries. Their bathymetric range is wide; many species are found in the intertidal area, but some extend to abyssal depths. They range considerably in length: in small species the length of the trunk may be only a few mm, whilst in the largest species the trunk may measure over 0.5 m. All are bottom dwellers and are rather sedentary in habit. Some live in sand or mud where they are activeburrowers, some in mucus-lined excavations. Others live in rock or coral crevices, in empty snail shells or annelid tubes. Sipunculids do not build true tubes.