Arrow worms are small, marine, planktonic animals of the phylum Chaetognatha that are found in tropical seas. Most of the 50 species belong to the genus Sagitta. Arrow worms have a head with eyes,
an elongated body—roughly the shape of an arrow— with two pairs of lateral fins and a tail fin. They use hook-like spines on their jaws to prey on smaller planktonic animals and larvae. Arrow worms are cross-fertilizing hermaphrodites; sperm from one individual is received by another in a sperm pouch, which later fertilizes the maturing eggs in the ovary.
Arrow worms are thought to be distantly related to the phylum Chordata (which includes the vertebrates), but they lack many 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. In arrow worms it forms as an out-pocket of the larval intestine. A similar organ is also found in the phylum Echinodermata and the subphylum Cephalochordata (Amphioxus) of the phylum Chordata. However, most chordates have a coelom that arises in a different way—by splitting tissue to form the cavity.