Chaetognatha

Chaetognatha, a class of small marine worms: they are all free swimming, and live as a rule on the surface of the sea far from land. A few species occur, however, in the seas around the British islands, and one (Spadella cephaloptera) is actually a shore-dweller, and fastens its eggs to algae. The class includes but two genera, Sagitta and Spadella. The main features in their anatomy are these: The body is divided by two septa into three regions. The mouth is a small slit on the ventral surface of the body: it is guarded by folds armed with spines and by series of hairs (setae) on the jaws. There are one or two lateral fins, and one on the tail. The anus is ventral, and opens at the junction of the body and tail. The nervous system consists of two ganglia, one anterior above the digestive tract, and one below it near the centre of the body. These are connected by series of nerves. There are two eyes. The Sagitta are hermaphrodite. The group is one of much interest, as it is small, and occupies a very isolated position in the great phylum of the Worms.