Chaetognatha

The Chaetognatha kiː'tɒgn@θ@ or chaetognaths ˈkiːtɒgnæθs (meaning bristle-jaws) are a phylum of predatory marine worms that are a major component of plankton worldwide. Commonly known as arrow worms, about 20% of the known Chaetognatha species are benthic, and can attach to algae and rocks. They are found in all marine waters, from surface tropical waters and shallow tide pools to the deep sea and polar regions. Most chaetognaths are transparent and are torpedo shaped, but some deep-sea species are orange. They range in size from . Chaetognaths were first recorded by the Dutch naturalist Martinus Slabber in 1775. As of 2021, biologists recognize 133 modern species assigned to over 26 genera and eight families. Despite the limited diversity of species, the number of individuals is large. Arrow worms are strictly related to and possibly belonging to Gnathifera, a clade of protostomes that do not belong to either Ecdysozoa or Lophotrochozoa. Chaetognaths are transparent or translucent dart-shaped animals covered by a cuticle. The body is divided into a distinct head, trunk, and tail. About 80% of the body is occupied by primary longitudinal muscles. There are between four and fourteen hooked, grasping spines on each side of their head, flanking a hollow vestibule containing the mouth. The spines are used in hunting, and covered with a flexible hood arising from the neck region when the animal is swimming. Spines and teeth are made of α-chitin, and the head is protected by a chitinous armature. The mouth opens into a muscular pharynx, which contains glands to lubricate the passage of food. From here, a straight intestine runs the length of the trunk to an anus just forward of the tail. The intestine is the primary site of digestion and includes a pair of diverticula near the anterior end. Materials are moved about the body cavity by cilia. Waste materials are simply excreted through the skin and anus. Eukrohniid species possess an oil vacuole closely associated with the gut.