44 



ORGANIC DEPENDENCE AND DISEASE 



the whole shell takes on the form of a smooth cone attached 

 by its apex. It is to be understood that the worm in these 

 cemented tubes was highly flexible and vibratile and free 

 to extend itself from the aperture and was not attached to 

 the tube shell ; and indeed, if like many living worms, could 



Fig. 16. Section of a Stromatopora colony showing the cut ends of the spiral 

 worm tubes Strepinclyfes concoetutttis from the Cobleskill limestone (Upper Silu- 

 rian). The apparent difference in direction of volution in these is entirely due 

 to difference of direction and angle at which the tubes are cut. 



Fig. 17. An enlarged restoration of the character of the worm tubes. 



Fig. 18. Streptindytes acervulariae Calvin. Two tubes of this spiral worm in a 

 colony of Acervularia Daridsoni. Middle Devonian, Iowa. 



abandon its shell entirely and build a new one somewhere 

 else. Streptindytes concoenatus is such a worm, with tube 

 stretched out in loose spiral, which we find to be common 

 in the Stromatopora colonies of the Upper Silurian (Coble- 

 skill) limestones. Our figures 16, 19, indicate that these 

 worms started their growth at different stages in the 

 growth of the colony, obviously attaching themselves to the 



