162 OUR COMMON BRITISH FOSSILS. 



and those which adopt a fixed existence. The former 

 have no hard parts to leave behind them when they 

 die, except their horny jaws, and these have been 

 found by Dr. Hinde in considerable quantities in the 

 Silurian and other rocks. Until a few years ago we 

 were entirely indebted for proofs of the former exist- 

 ence of this class of sea-worms to the tracks they 

 left behind them on primaeval mud-flats, and the 

 discovery of their jaws is additional evidence in favour 



Fig. 129. Horny Jaws of modern marine Annelida. 



of these tracks being of annelid origin. The other 

 class of sea-worms, adopting a settled marine life, live 

 in tubes, which are formed of grains of sand, etc., 

 cemented together, as in the instance of our modern 

 Terebella ; or they may be leathery, as in the recent 

 Sabella; or their outer skin may secrete lime, and 

 thus form a solid tube, as illustrated by the empty 

 tubes we see attached to old oyster-shells, stones, and 

 rocks. In both the latter cases the breathing organs 

 are gathered into one place, and form a beautiful 

 feathery tuft, sometimes brightly coloured, as in those 

 of that little coiled worm which rejoices in such a high 

 antiquity, the Spirorbis. 



