Chaetognatha

Chaetognatha, 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, and the Middle Cambrian Burgess Shale of British Columbia.
These animals are usually considered a type of protostome.