The Oligochæta form a division of the Annelida, of which the most familiar type is the common earthworm (Lumbricus terrestris); the group comprises also a great number of smaller worms, which are for the most part inhabitants of ponds and streams, such as the red River worm (Tubifex rivulorum). The Oligochæta were at one time believed to be entirely terrestrial or inhabitants of fresh water, and to be distinguished thus from the Polychæta, which were supposed to be exclusively marine in their habitat. Although the progress of research has not broken down the structural distinctions between these two divisions of the Annelida chætopoda, it has been provedthat no absolute line of demarcation can be drawn between the Oligochæta and the Polychæta as regards their habitat; on the one hand Polychæta have been found in fresh water, and, on the other hand, certain species of Oligochæta are now known to inhabit the mud and gravel of the seashore.