Chaetifera

Chaetifera, one of the two divisions of the Gephyrea class of Worms. This order seems the more primitive of the two, and if so it strongly supports the view of the Annelidan origin of this class. The Chaetifera are, at least in early stages, composed of several segments; they have a long proboscis (or "prostomium") which is protrusible at the anterior end of the body; the mouth opens at the junction of this with the body. There is one pair of setae which may represent the last of a series which have been lost. The anus is terminal. and the hindmost pair of excretory organs or "nephridia" open into the rectum close by the anus. There are several pairs of anterior nephridia. Thus the segmentation of the ancestral form is indicated by three points: the external segmentation, the pair of setae, and the pairs of nephridia. In some cases the embryro is small, and resembles a minute Planarian (q.v.).