Arrow worms are small marine planktonic animals of the phylum Chaetognatha found in tropical seas. Most of
the 50 species of arrow worms belong to the genus Sagitta. Arrow worms have a head with eyes and hook-like spines on their jaws that identify them as predators of smaller planktonic animals and larvae. Arrow worms have an elongated body, roughly the shape of an arrow with two pairs of lateral fins and a tail fin. They are cross-fertilizing hermaphroditic animals. Sperm from one individual is received by another individual in a sperm pouch, which later fertilizes the maturing egg in the ovary.
Arrow worms are thought to be distantly related to the phylum Chordata (which includes the vertebrates) but they lack many of the other important chordate characteristics. Nevertheless, arrow worms do have a coelom (a fluid-filled body cavity) which is a characteristic of chordates and the phylum Protochordata. The coelom in arrow worms forms as an out-pocketing of the larval intestine. A similar origin of the coelom is also found in the phylum Echinodermata and the subphylum Cephalochordata (Amphioxus) of the phylum Chordata. However, the majority of chordates (the vertebrates in the subphylum Vertebrata) have a coelom that arises in a different way—by a splitting of the tissues to form the cavity.