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 Strepindytes concoenatus 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 Aeervularia Davidsoni. 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 
