Members of the phylum Annelida include the segmented worms such as the
common earthworm. Annelids are marine, fresh water, or terrestrial and
have apparently been common through much of geologic time. Because of
their lack of hard parts, most of these worms have left little direct
fossil evidence of their activities in the geologic past. Some annelids
secrete straight or coiled calcareous tubes, and fossil worm tubes of
this sort (fig. 20) are commonly found attached to brachiopods,
mollusks, and other objects. Tubes of this nature have been reported
from Paleozoic, Mesozoic, and Cenozoic rocks in Texas.
[Illustration: Fig. 20. Types of typical annelid worms. (a) _Serpula_
(×1) (b) _Hamulus_ (×2). (c) _Spirobis_ (×15).]
Some annelids have small chitinous jaws and teeth which also may be
preserved as fossils. These dental structures are called scolecodonts
and are microfossils.