An Arrow Worm.
 
Small (< 10 cm.) predaceous worms that live in the plankton. The body is divided into three parts as shown. The spines, in conjunction with teeth (not shown) that surround the mouth, are used to grasp and ingest prey. One species, sagitta elegans, has been known to consume young fish and other arrow worms its own size. Attack and dispatch of victims is accomplished with lightening speed. Locomotion is via contraction of the longitudinal muscles which causes the animal to dart forwards after which it glides and floats. The fins are used solely flotation. From Barnes, R. D. 1964. Invertebrate Zoology. W. B. Saunders. Philadelphia.