The Chaetognath Sagitta (image) Okinawa Institute of Science and Technology (OIST) Graduate University Share Print E-Mail Caption Chaetognaths, or arrow worms, have a distinct jaw structure composed of dense protein matrix and a fibrous substance called chitin. These organisms display an ambiguous set of developmental and morphological features, making them difficult to categorize on the Tree of Life. Credit Creative Commons Usage Restrictions CC2.0 [attribution] Share Print E-Mail Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.