Chaetognatha, meaning hair-jaws, and commonly known as arrow worms, are a phylum of predatory marine worms that are a major component of plankton worldwide. About 20% of the known species are benthic, that is belonging to the lowest zone of the ocean, or benthic zone, 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 2 to 120 millimetres (0.079 to 4.7 in).
There are more than 120 modern species assigned to over 20 genera. Despite the limited diversity of species, the number of individuals is large.
Chaetognaths are transparent or translucent dart-shaped animals covered by a cuticle. The body is divided into a distinct head, trunk, and tail. 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. All chaetognaths are carnivorous, preying on other planktonic animals.
The trunk bears one or two pairs of lateral fins incorporating structures superficially similar to the fin rays of fish. Unlike those of vertebrates, however, these are composed of a thickened basement membrane extending from the epidermis, and they are not homologous. An additional caudal fin covers the post-anal tail.  At least one species of chaetognath, Caecosagitta macrocephala, has bioluminescent organs on its lateral fins.
Chaetognaths swim in short bursts using a dorso-ventral undulating body motion, where their tail fin assists with propulsion and the body fins for stabilization and steering. Some species are known to use the neurotoxin tetrodotoxin to subdue prey.
The body cavity lacks a peritoneum, and therefore resembles the pseudocoel of animals such as nematodes, but is divided into one compartment on each side of the trunk, and additional compartments inside the head and tail. Although they have a mouth with one or two rows of tiny teeth, compound eyes, and a nervous system, they have no respiratory or circulatory systems.
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 in front of the tail. The intestine is the primary site of digestion and includes a pair of diverticula near the anterior end.
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