A phylum of elongated, often flattened marine worm-like animals comprising nearly 1000 species which range in size from less than an inch to nearly 30 m. They are carnivorous and feed upon annelid worms as well as other marine animals. Their most distinctive structure is a proboscis which is used for food capture, defense, and burrowing into the soft substrate. The sexes are separate in most species and fertilzation is external. Many are capable of reproducing asexually by fragmentation of the body. They are also called "ribbon worms".