SILURIAN WORM TRACKS. 



175 



especially on the coast of Brazil. The minute nautilus-like 

 shells of Spirorbis live attached to the fronds of sea-weeds, 

 especially the different kinds of Fucus. 



Fig. 124.Cirratitlus grandis. After Verrill. 



Many sea-worms are highly phosphorescent, the light emit- 

 ted being intensely green. The tracks of worms like the 

 Nereis of to-day occur in the lower Silurian slates ; their 

 bristles, however, were spinulose, as in the larval worms. 

 Thus the type, though highly specialized, has, unlike most 

 specialized groups, a high antiquity, the specialized Anne- 

 lides existing side by side with the generalized Polyzoa and 

 Bracliiopoda. At the present time the Annelides are Avidely 

 distributed in the seas of the globe, the tropical forms being 

 exceedingly abundant among coral stocks and in sponges, 

 while the arctic seas abound with Annelid life. They also 

 sparingly exist at great depths, one species of a worm allied 



