A phylum of abundant planktonic arrowworms. As adults they range in size from 0.2 to 4 in. (5 to 100 mm). Their bodies are tubular and transparent, and divided into three portions: head, trunk, and tail (illus. a). The head possesses one or two rows of minute teeth anterior to the mouth and usually 7–10 larger chaetae, or seizing jaws, on each side of the head. One or two pairs of lateral fins and a caudal fin are present. In mature individuals a pair of seminal vesicles protrudes from the sides of the tail segment just forward of the caudal fin.