The arrow-worms are a most unusual group of “worms” occurring only in the marine plankton, and related to no other worms, and to no other known group of animals. The Greek name Chaetognath means “bristle-jaw” after the sharp and business-like feeding apparatus around the head, while one of the commonest species, Sagitta, is literally “arrow”. It is difficult to represent their true appearance because, apart from two tiny black eyes, they are completely transparent, and in fact the fins are almost completely invisible except under a phase contrast microscope. Perhaps this explains why the group is not commonly known. This animal is quite large (up to 6 in. in length) and, next to the copepods, is the commonest of all the plankton. Sagitta is in proportion to its size even more predacious than the meanest maneating shark and has been known to attack and swallow whole fish as large as itself. Owing to the “worm's” transparency, it is possible to watch the whole digestive process under the microscope. Two species of Sagitta, S. setosa and S. elegans, are famous in the marine biological world because of their value in the English Channel and North Sea as “indicator species”–that is, the kind of Sagitta indicates whether the water is ordinary coastal water (S. setosa) or an oceanic influx from the Atlantic (S. elegans). Since the same group of species occur in New Zealand waters, they may well prove of value in studying our own ocean currents.
by Richard Morrison Cassie, M.SC.(N.Z.), D.SC.(AUCK.), Senior Lecturer in Zoology, University of Auckland.