ORGANIC DEPENDENCE AND DISEASE 



41 



cially with the corals and the sponges and the calcareous 

 algae. 



The coexistence of the tubicolous worms Avith the corals 

 is one of the commonest phenomena of present seas and it 

 became established as early as the Silurian. In most of 

 the ancient cases observed it is an elementary expression 

 of commensalism, but not long after its start it becomes at 

 times rather complex. Worm and coral may start together 

 directly on settling down from the free larval state, or con- 

 junction may be formed by attachment of the annelid larva 

 after the growth of the coral has well progressed. In both 

 cases the growth of the latter engulfs the former save at 

 its tentacled aperture. We give herewith examples of these 

 occurrences.^ 



Fig. 3. The coral Cystiphyllum with short tubes of GHonia corattopliila opening 



outward through the thecal walls. 

 Fig. 4. A calyx of Zaphrentis with a number of tube openings of Gitonia. 

 Figs. 5, 6. A Zaphrentis from two points of view to show the course of the tube 



of 6r. corallopMla with both ends opening outward into the calyx. 

 Fig. 7. Tubes of this character opening through the lateral walls of Zaphrentis. 

 All are from the Onondaga limestone (Lower Devonian). 



1 Some of these illustrations are taken from the writer 's ' ' Beginnings of 

 Dependent Life" (1908), but to these and to the other classes discussed, new 

 illustrations have been added. 



