ANNELIDA. CH^TOPODA 159 



the polychsetous worms are often found fossil. Other 

 forms, which do not live in tubes, are provided with 

 minute chitinous jaws, and in some formations, especially 

 the Ordovician and Silurian, these are abundantly pre- 

 served. 



Serpula. Tube calcareous, long, round, angular or flattened ; 

 straight, curved irregularly or sometimes spirally, closed at one 

 end ; generally attached to some foreign object by a portion of its 

 surface. Silurian to present day. Ex. S. gordialis, Chalk. 



Spirorbis. Tube calcareous, small, spiral, attached by one side. 

 The spiral either left-handed or right-handed, the last whorl often 

 produced into a free tube. Ordovician to present day. Ex. S. 

 pusillus ( = carbonarius), Carboniferous. 



Distribution of the Chcetopoda 



Nearly all the worms which are found fossil belong to 

 the Polychaeta; the earliest examples occur in the Cambrian 

 Beds. In addition to worm-tubes and jaws, there are, in 

 various rocks, numerous trails and burrows, which are con- 

 sidered by some authors to have been formed by worms, 

 but in many cases it is probable that they were made by 

 other animals such as crustaceans and gasteropods. 



