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 
12 
Figs. 12-15. Enlarged drawings of Spirorbis angulatus, 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. 
