Chaetognatha

Chaetognatha,meaning hair-jaws commonly known as arrow worms, are a phylum of small predatory marine animals. They are present in huge numbers in plankton worldwide.

About 20% of the known species are benthic and can attach to algae or 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 .

Despite the huge numbers, there are only about 120 modern species in 20 genera. Some species are known to use the neurotoxin tetrodotoxin to subdue their prey.  

Chaetognaths appear to have originated in the Cambrian Period. Complete body fossils have been described from China,Chen J.-Y., and D.-Y. Huang. 2002. A possible Lower Cambrian chaetognath (arrow worm). Science 298:187.Hu S.-X. 2005. Taphonomy and palaeoecology of the Early Cambrian Chengjiang Biota from Eastern Yunnan, China. Berliner Paläobiologische Abhandlungen 7:1–197. Greenland    and the Middle Cambrian Burgess Shale of British Columbia.Szaniawski H. 2005. Cambrian chaetognaths recognized in Burgess Shale fossils. Acta Palaeontologica Polonica 50:1-8.

These animals are usually considered a type of protostome, closely related to rotifers.
 References 

Category:Minor phyla
Category:Invertebrates