Arrow worm. Scanning electron micrograph (SEM) of an arrow worm (phylum Chaetognatha). This microscopic predatory worm is transparent or translucent and is covered by a cuticle. It has fins and a pair of hooked, chitinous, grasping spines (blue) on each side of its head, which are used in hunting. The spines are covered with a hood when swimming. Arrow worms are hermaphroditic, carrying both eggs and sperm. Some species are known to use the neurotoxin tetrodotoxin to subdue prey. Magnification: x100 when printed 10 centimetres wide.