THE HYDRA AND ITS ALLIES 



215 



to New Jersey, but reaches its maximum development 

 along the coast of Maine. It varies greatly in color, some 

 individuals being white, others salmon-colored or olive. 

 The flat upper surface bears the slit-like opening to the 

 internal sac. Either one or both angles of this slit have 

 thickened edges, and corresponding to this difference is a 



FIG. 202. Metridium, one of our sea-anemones. Two individuals shown 

 expanded. Photo, of the living animals in the water, by W. H. C. P. 



difference in the internal structure. Inside, the central 

 cavity is separated into compartments by radial partitions. 

 The coral polyp does not differ essentially from Metri- 

 clium. But it has the habit of secreting lime at its base, 

 so that, in course of time, a high cup is built up. The top 

 of the cup bears radial septa, 1 which are laid down 



i Fig. 203. 



