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. compactus, 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. 



Fig. 19. Mreptiudytes compactm, 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-shaped colonies with large cells, in its struc- 

 ture very similar to the common honeycomb coral Favo- 



