These protective tubes become encrusted with shells or are buried in sand. Horseshoe worm.... horsesho worm

Horseshoe worms, or phoronids, either are solitary or occur in groups of many thousands of individuals; one species is colonial and buds asexually. Although most horseshoe worms are found intertidally or in shallow ocean regions in temperate zones, their habitats also include tropical regions and ocean depths. In general, horseshoe worms are not common; certain species, however, may carpet the bottoms of shallow bays. Since their discovery in 1846, horseshoe worms have been associated with every animal phylum, yet the relationship of horseshoe worms to other animals remains controversial. They have not been preserved in the geological record, although fossil tubes (called Scolithus ) may belong to horseshoe worms. Embryological stages often are useful in relating animal groups, but the actinotroch larva of horseshoe worms is unique. horsesho worm