t 



lr.4 



EXPLANATION OF PLATE XXI. 



Fig. 1. Polypidom of Coppinia mirabilis, in its perfect condition, showing the polype- 

 cells, matrix, and opercula. 



Fig. 2. A variety in the polypidom of Coppinia mirabilis, in which the polype-cells 

 are much elongated and twisted, and the matrix less regular and perfect. 



Fig. 3. Polypidom and overgrown vesicles of Sertularia gracilis. 



Fig. 4. Polypidom of Campanularia serpens, parasitic on Sertularia abietina. 



