152 THE INFLUENCE OF INANIMATE SURROUNDINGS. 



live in pure fresh water. When I was still a student, Cor- 

 fjylophora lacustris (fig. 40) was found only in estuaries and at 

 the mouths of rivers where the water was at any rate occa- 

 sionally salt or brackish; it was discovered almost simul- 

 taneously in England and Belgium, and somewhat later I 

 found it in the Schlei, near Schleswig. Since that time, 1854, 

 the animal has in many places migrated into rivers ; it has 

 already reached the Seine at Paris, and has got into the fresfy- 



FIG. 40; Cordylophora lacustris (from J*. E. Schnltze), a brackish-water polyp which 

 within the last ten years lias gradually migrated into pure fresh water. 



water aquarium of the Jardin des Plantes there, where it ' is 

 said to be very common. Its migrations in the Elbe were still 

 more remarkable. After reaching Hamburg, and even, if I am 

 not mistaken, finding its way into the Alster, it took possession 

 at the same time of the great water-pipes of the city, in which 

 it lived, associated with the well-known bivalve, Dreissena 

 polymorpha, in such enormous quantities as to impede the flow 

 of water through the pipes. This case is the more interesting 



