160 PROCEEDINGS OF THE ACADEMY OF [1897. 



Figure 16. Later shell secreting stage. Ventral view. 



Figure 17. Adult Spirorbis borealw showing arrangement of 



branchiae, operculum, alimentary tract, shell gland, 

 generative glands. 



Figure 18. Cephalic and anterior thoracic regions of adult S. 



borealis showing arrangement of branchiae and 

 operculum, groups of setse, fore-gut of alimentary- 

 tract. 



