Polygordins, the type-genus of the family Polyyordiidce, and the best-known member of the class of primitive worms known as the Archi-Annelida (q.v.). It is a small marine worm, with the segmentation very feebly marked; they have neither bristles (setas), lateral appendages (parapodia), nor the respiratory processes known as cirri and branchiae. The intestine is straight, and the kidneys (nephridia) also very simple. The fact that the nerves are retained in the hypodermis is another very primitive character. There are several species which are common in the Mediterranean; when irritated, they break voluntarily into fragments.