Archiannelida

Archiannelida, an order of Annelida, including the families Histriodrilidae and Polygordiidae; they all are marine and their characters are very primitive. Thus the nervous system is retained as two threads on the sides of the body, instead of passing inwards and downwards, forming a double chain on the under or ventral side. The nephridia, or organs of excretion, are simple. The division of the body into "segments" or rings is feebly marked; most of these segments are alike, but the first segment (prostomium, i.e. before the mouth) is small, and the second (peristomium, i.e. around the mouth, which is situated in this segment) large. The head cavities are true Archicoeles (q.v.).