ORGANIC DEPENDENCE AND DISEASE 43 



place and serves the purpose of a self-constructed tube. 

 Thus the worm Gitonia corallophila expresses itself in vari- 

 ous meanderings among the simple corals. Some small 

 lens-shaped coral colonies from the Ordovician of Iowa are 

 permeated with worm associates, all of which seem to start 

 from the initial basal point of growth of the coral, and 

 then, after a single turn or so of the tube in Serpula fashion, 

 strike outward radially between the polyp cells, all reach- 

 ing the tentacle surface of the colony. This combination 

 indicates that the embryo worms aggregated themselves in 

 numbers about the anchoring coral larva. 



Spiral worms and corals. These interesting associations 

 are common throughout the Silurian and Devonian. Spiral 

 worm tubes passing in these faunas under the name of 

 Spirorbis and living independently are normally, or at 

 least often, attached to shells of brachiopods and mollusks, 

 where they escape any chance of becoming embedded, and 

 after a few initiatory attached coils the tube often becomes 

 free and resolves itself into very loose spirals (see figures 

 of S. angulatus}. In the tube called Autodetus, which is 

 frequent in the Devonian, there is an initial spiral attach- 

 ment, but the whorls of the free tube keep in contact and 



Figs. 12-15. Enlarged drawings of Spirorbis anyuUtus, a worm tube from the 

 Hamilton group (Middle Devonian). These show the tendency of the tube to 

 unwind in a lax spiral as soon as fixation is firmly established. 



