the free-swimming larva of worms of the order Heteronemertina of the phylum Nemertea. The body is 1 to 4 mm long and covered with cilia. A band of long cilia, the proto-troch, extends around the base and is drawn out into lateral lobes. On the apical portion of the pilidium there is a bundle of cilia, which serves as a sensitive parietal organ. The mouth opening is located on the lower side of the body and leads to the gut. The animal swims in deep water with the aid of the prototroch. The body of the worm forms from the internal part of the larva and small parts of its ectodermal plates, or disks. The rest of the larval body plays no role in the formation of the worm. The young nemertine descends to great depths and spends its life crawling along the bottom.