Editing Chaetognatha (section)

===Internal organs===
The body cavity is lined by [[peritoneum]], and therefore represents a true [[coelom]], and is divided into one compartment on each side of the trunk, and additional compartments inside the head and tail, all separated completely by septa.  Although they have a mouth with one or two rows of tiny teeth, compound eyes, and a nervous system, they have no excretory or respiratory systems.<ref>[https://books.google.com/books?id=9IWaqAOGyt4C&dq=Chaetognatha+circulatory+excretory+respiratory+systems&pg=PA343 Kingdoms and Domains: An Illustrated Guide to the Phyla of Life on Earth]</ref><ref name="Perez2021"/> While often said to lack a circulatory system, chaetognaths do have a rudimentary hemal system resembling those of [[Annelida|annelids]].<ref name="Perez2021"/>

The arrow worm [[rhabdomere]]s are derived from [[microtubule]]s 20&nbsp;nm long and 50&nbsp;nm wide, which in turn form conical bodies that contain granules and thread structures. The cone body is derived from a [[cilium]].<ref>"Photoreception". Encyclopædia Britannica from [[Encyclopædia Britannica 2006 Ultimate Reference Suite DVD]] . 2009.</ref>