English: Arrow Worms.
Genera: 20; over 120 species.
Habitat: all marine waters from surface tropical waters and shallow tide pools to the deep sea and polar regions.
ZoologyChaetognatha are predatory marine worms that are a major component of plankton worldwide. About 20% of the known species are benthic and can attach to algae or rocks. Most chaetognaths are transparent and are torpedo shaped. Some deep-sea species are orange. They range in size from 2 mm to 12 cm. They are covered by a cuticle. They have fins and a pair of hooked, chitinous, grasping spines on each side of their heads that are used in hunting. The spines are covered with a hood when swimming. They have a distinct head, trunk and tail. All species are hermaphroditic, carrying both eggs and sperm. Some species are known to use the neurotoxin tetrodotoxin to subdue prey. They have a mouth with one or two rows of tiny teeth, compound eyes, and a nervous system. They have no respiratory, circulatory, or excremental systems. Materials are moved about the body cavity by cilia. Waste materials are simply excreted through the skin and anus. They 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.
TaxonomyThey have some developmental similarities to nematodes.