ORGANIC DEPENDENCE AND DISEASE 
45 
outer surface of the coral when it was well grown, but it 
is interesting to see that at any given stage they attach 
themselves not singly but in numbers, as though each set- 
tlement indicated a new crop of young worms. Streptin- 
dytes acervulariae Calvin is a quite large spiral tube not 
uncommon in Acervularia davidsoni, a coral of the Middle 
Devonian of Iowa, and S. comp actus , a short, close-coiled 
species which is found buried up in the calcareous sub- 
stance of the Iowa Middle Devonian Stromatoporas. These 
embedded worms were often eventually strangled by the 
more rapid overgrowth of the coral, as there was no lateral 
way out for their heads as in the straight tubes. 
19 
Fig 1 . 19. Streptindytes compact" s , a spiral worm embedded in a solid stromatopo- 
roid coral. Sections of the tubes are indicated by the white dots. 
Fig. 20. A single individual enlarged. From the Middle Devonian of Iowa. 
The extraordinary case of the coral Pleurodictyum and 
its commensals. Pleurodictyum is a small compound coral 
growing in lens-sliaped colonies with large cells, in its struc- 
ture very similar to the common honeycomb coral Favo- 
