Chaetognatha

A phylum (Chaetognatha) of predatory marine worms that are a major component of plankton worldwide. They show some preference for warmer waters. Chaetognaths are transparent and are torpedo shaped sometimes with arrowhead like opaque structures in their heads. They range in size from 3mm to 12cm. The common term for the phylum is "Arrow Worms". There are about 100 modern species assigned to 15 genera. Despite the limited diversity, the number of individuals is staggering.

Chatognaths are transparent or translucent and 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. A few species are known to use neurotoxins to subdue prey. Chaetognaths are traditionally classed as deuterostomes by embryologists. Molecular Phylogenists, however, consider them to be protostomes. They have some developmental similarities to nematodes. Although they have a mouth with one or two rows of tiny teeth, compound eyes, and a nervous system, they have no repiratory, circulatory, or excremental systems. Materials are moved about the body cavity by cilia. Waste materials are simply excreted through the skin.

Chaetognaths swim using their tail fin for propulsion and the body fins for stabilization and steering.

Chaetognaths fossilize poorly, but are thought to have originated in the Cambrian. Chaetognath grasping spines are found occasionally as fossils from the late Paleozoic onward. Complete body fossils that have not been formally described are reported from the Kicking Horse Shale member of the Middle Cambrian Burgess Shale of British Columbia and the Lower Cambrian Maotianshan shales of Yunnan, China. Chaetognaths are thought possibly to be related to some of the animals grouped with the conodonts. The conodonts themselves, however, are thought to be related to the vertebrates. It is now thought that the protoconodonts[?], which are known only from their teeth, might be chaetognaths rather than conodonts. The Burgess Shale fossil Amiskwia is thought by some to be a Chaetognath, but it lacks teeth and is generally thought to belong to some other phylum of worms.

http://courses.washington.edu/emb2002/resources/lecturereferences/07052002/chaetognatha.pdf http://citd.scar.utoronto.ca/EESC04/SCMEDIA/INVPHYLO/Chaetognatha/GenInfo http://depts.washington.edu/fhl/zoo432/plankton/plchaetognatha/chaetognatha